I am not a medical professional, but my wife is a hospital phamacist. I asked her and she said, predictably enough, ask your dispensing pharmacist to see if the combination of those three particular medications is known to be safe in the manner you wish to administer them.
The safety of mixing drugs, I am told, is dependent on a vast number iof factors, including the drugs themselves, the suspending agents, sweeteners or flavouring agents, etc. Some things might never react, others might do so instantly upon contact.
The wife says that there may have been some study of these particular medications for safety and effectiveness if mixed before use, but she kind of doubts it. Those kinds of studies are much more common with IV drugs. The advice she usually gives to doctors and nurses when asked about mixing IV’s (and advice she thinks is good for this situation as well) is “If there isn’t a study that says it’s OK, assume it isn’t and don’t do it.”