Stay with it. After Rose you do feel a bit let down and like Martha is a kind of poor substitute. I imagined the Doctor feeling the same way. But by god, you stick with it through Donna at least.
I hated Donna Noble the first couple times I saw her but I started warming to her by the end of The Fires of Pompeii (har har har). I hated her so much I wasn’t sure I’d continue watching. Family of Blood is one of my favorites - you get to see a side of the Doctor that is truly a bit frightening.
So no, it’s not downhill from here. It’s just a little weird and bumpy and different. Wibbly, wobbley, timey wimey, if you will. Don’t blink.
I’m a huge Dr. Who nerd, and I’ve seen, well, pretty much all of them. To my mind, Human Nature/Family of Blood is the best story in the show’s almost 50 year long history. And Blink is an extremely close second. Stick with it.
I didn’t mind Martha so much, and I think she was redeemed in the end. I have to add that “The Sound of the Drums” is probably my favorite episode from Series 3. Derek Jacobi was amazing.
I lucked out - I missed the first half of Season 3 its first time through, and missed most of the terribleness. As such, I thought Martha was an incredibly strong companion (which she is - it’s just that that’s her stereotype, she’s “The Strong One”). I was mostly annoyed by Donna, until I rewatched her episodes a second time - you really get to watch her become The Doctor’s best friend forever from a new perspective.