If it gets to that point use a computer.
For example this line of Mathematica:
Partition[ Times @@ # & /@
Subsets[{a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r}, {3}], 24]
Produces exactly the three letter words you want and places them in a 34 by 24 table in lexicographic order.
Full output follows:
{{a b c, a b d, a b e, a b f, a b g, a b h, a b i, a b j, a b k, a b l, a b m, a b n, a b o, a b p, a b q, a b r, a c d, a c e, a c f, a c g, a c h, a c i, a c j, a c k},
{a c l, a c m, a c n, a c o, a c p, a c q, a c r, a d e, a d f, a d g, a d h, a d i, a d j, a d k, a d l, a d m, a d n, a d o, a d p, a d q, a d r, a e f, a e g, a e h},
{a e i, a e j, a e k, a e l, a e m, a e n, a e o, a e p, a e q, a e r, a f g, a f h, a f i, a f j, a f k, a f l, a f m, a f n, a f o, a f p, a f q, a f r, a g h, a g i},
{a g j, a g k, a g l, a g m, a g n, a g o, a g p, a g q, a g r, a h i, a h j, a h k, a h l, a h m, a h n, a h o, a h p, a h q, a h r, a i j, a i k, a i l, a i m, a i n},
{a i o, a i p, a i q, a i r, a j k, a j l, a j m, a j n, a j o, a j p, a j q, a j r, a k l, a k m, a k n, a k o, a k p, a k q, a k r, a l m, a l n, a l o, a l p, a l q},
{a l r, a m n, a m o, a m p, a m q, a m r, a n o, a n p, a n q, a n r, a o p, a o q, a o r, a p q, a p r, a q r, b c d, b c e, b c f, b c g, b c h, b c i, b c j, b c k},
{b c l, b c m, b c n, b c o, b c p, b c q, b c r, b d e, b d f, b d g, b d h, b d i, b d j, b d k, b d l, b d m, b d n, b d o, b d p, b d q, b d r, b e f, b e g, b e h},
{b e i, b e j, b e k, b e l, b e m, b e n, b e o, b e p, b e q, b e r, b f g, b f h, b f i, b f j, b f k, b f l, b f m, b f n, b f o, b f p, b f q, b f r, b g h, b g i},
{b g j, b g k, b g l, b g m, b g n, b g o, b g p, b g q, b g r, b h i, b h j, b h k, b h l, b h m, b h n, b h o, b h p, b h q, b h r, b i j, b i k, b i l, b i m, b i n},
{b i o, b i p, b i q, b i r, b j k, b j l, b j m, b j n, b j o, b j p, b j q, b j r, b k l, b k m, b k n, b k o, b k p, b k q, b k r, b l m, b l n, b l o, b l p, b l q},
{b l r, b m n, b m o, b m p, b m q, b m r, b n o, b n p, b n q, b n r, b o p, b o q, b o r, b p q, b p r, b q r, c d e, c d f, c d g, c d h, c d i, c d j, c d k, c d l},
{c d m, c d n, c d o, c d p, c d q, c d r, c e f, c e g, c e h, c e i, c e j, c e k, c e l, c e m, c e n, c e o, c e p, c e q, c e r, c f g, c f h, c f i, c f j, c f k},
{c f l, c f m, c f n, c f o, c f p, c f q, c f r, c g h, c g i, c g j, c g k, c g l, c g m, c g n, c g o, c g p, c g q, c g r, c h i, c h j, c h k, c h l, c h m, c h n},
{c h o, c h p, c h q, c h r, c i j, c i k, c i l, c i m, c i n, c i o, c i p, c i q, c i r, c j k, c j l, c j m, c j n, c j o, c j p, c j q, c j r, c k l, c k m, c k n},
{c k o, c k p, c k q, c k r, c l m, c l n, c l o, c l p, c l q, c l r, c m n, c m o, c m p, c m q, c m r, c n o, c n p, c n q, c n r, c o p, c o q, c o r, c p q, c p r},
{c q r, d e f, d e g, d e h, d e i, d e j, d e k, d e l, d e m, d e n, d e o, d e p, d e q, d e r, d f g, d f h, d f i, d f j, d f k, d f l, d f m, d f n, d f o, d f p},
{d f q, d f r, d g h, d g i, d g j, d g k, d g l, d g m, d g n, d g o, d g p, d g q, d g r, d h i, d h j, d h k, d h l, d h m, d h n, d h o, d h p, d h q, d h r, d i j},
{d i k, d i l, d i m, d i n, d i o, d i p, d i q, d i r, d j k, d j l, d j m, d j n, d j o, d j p, d j q, d j r, d k l, d k m, d k n, d k o, d k p, d k q, d k r, d l m},
{d l n, d l o, d l p, d l q, d l r, d m n, d m o, d m p, d m q, d m r, d n o, d n p, d n q, d n r, d o p, d o q, d o r, d p q, d p r, d q r, e f g, e f h, e f i, e f j},
{e f k, e f l, e f m, e f n, e f o, e f p, e f q, e f r, e g h, e g i, e g j, e g k, e g l, e g m, e g n, e g o, e g p, e g q, e g r, e h i, e h j, e h k, e h l, e h m},
{e h n, e h o, e h p, e h q, e h r, e i j, e i k, e i l, e i m, e i n, e i o, e i p, e i q, e i r, e j k, e j l, e j m, e j n, e j o, e j p, e j q, e j r, e k l, e k m},
{e k n, e k o, e k p, e k q, e k r, e l m, e l n, e l o, e l p, e l q, e l r, e m n, e m o, e m p, e m q, e m r, e n o, e n p, e n q, e n r, e o p, e o q, e o r, e p q},
{e p r, e q r, f g h, f g i, f g j, f g k, f g l, f g m, f g n, f g o, f g p, f g q, f g r, f h i, f h j, f h k, f h l, f h m, f h n, f h o, f h p, f h q, f h r, f i j},
{f i k, f i l, f i m, f i n, f i o, f i p, f i q, f i r, f j k, f j l, f j m, f j n, f j o, f j p, f j q, f j r, f k l, f k m, f k n, f k o, f k p, f k q, f k r, f l m},
{f l n, f l o, f l p, f l q, f l r, f m n, f m o, f m p, f m q, f m r, f n o, f n p, f n q, f n r, f o p, f o q, f o r, f p q, f p r, f q r, g h i, g h j, g h k, g h l},
{g h m, g h n, g h o, g h p, g h q, g h r, g i j, g i k, g i l, g i m, g i n, g i o, g i p, g i q, g i r, g j k, g j l, g j m, g j n, g j o, g j p, g j q, g j r, g k l},
{g k m, g k n, g k o, g k p, g k q, g k r, g l m, g l n, g l o, g l p, g l q, g l r, g m n, g m o, g m p, g m q, g m r, g n o, g n p, g n q, g n r, g o p, g o q, g o r},
{g p q, g p r, g q r, h i j, h i k, h i l, h i m, h i n, h i o, h i p, h i q, h i r, h j k, h j l, h j m, h j n, h j o, h j p, h j q, h j r, h k l, h k m, h k n, h k o},
{h k p, h k q, h k r, h l m, h l n, h l o, h l p, h l q, h l r, h m n, h m o, h m p, h m q, h m r, h n o, h n p, h n q, h n r, h o p, h o q, h o r, h p q, h p r, h q r},
{i j k, i j l, i j m, i j n, i j o, i j p, i j q, i j r, i k l, i k m, i k n, i k o, i k p, i k q, i k r, i l m, i l n, i l o, i l p, i l q, i l r, i m n, i m o, i m p},
{i m q, i m r, i n o, i n p, i n q, i n r, i o p, i o q, i o r, i p q, i p r, i q r, j k l, j k m, j k n, j k o, j k p, j k q, j k r, j l m, j l n, j l o, j l p, j l q},
{j l r, j m n, j m o, j m p, j m q, j m r, j n o, j n p, j n q, j n r, j o p, j o q, j o r, j p q, j p r, j q r, k l m, k l n, k l o, k l p, k l q, k l r, k m n, k m o},
{k m p, k m q, k m r, k n o, k n p, k n q, k n r, k o p, k o q, k o r, k p q, k p r, k q r, l m n, l m o, l m p, l m q, l m r, l n o, l n p, l n q, l n r, l o p, l o q},
{l o r, l p q, l p r, l q r, m n o, m n p, m n q, m n r, m o p, m o q, m o r, m p q, m p r, m q r, n o p, n o q, n o r, n p q, n p r, n q r, o p q, o p r, o q r, p q r}}
You should be able to cut and paste that into notepad and use find and replace to turn it into something that you can then paste into an Excel worksheet.
