I could be wrong, but I'm thinking 3 months is too long. That's 90 days of age, most bottle calves have been weaned for over a month since then, on grain, hay, over the usual stresses of weaning, etc. My feeling is that most of your problems are going to be in the first 30 days, and by the time they pass 60 days they're pretty much in the clear. That's the age that a lot of folks want them because there's no bottle feeding, less chance of death, calves know how to eat, etc etc. I personally don't mind the challenge of taking them on earlier and I've done alright.
All the bottle calves I've gotten except one had two feedings of colostrum before I picked them up, and I'll pick up my bottle calves as early as 12 hours of age. The one calf I got that hadn't had colostrum...she has a story of her own. No colostrum or even regular milk in the first 24 hours (too long to explain why), came down with pnemonia by 3 days (perhaps more related to the cold January weather?), and I picked her almost-literally off the dead animal pile at 5 days of age.
A lot of TLC, and she survived. Pushing 700lbs now. ;-) I personally don't feel a lack of colostrum is a death sentence; perhaps a setback at first, but not a death sentence.
What took your calves down? respiratory?
Not trying to brag here, but out of quite a few bottle calves, I've only lost one, and she was another I pulled off the dead animal pile - this time with scours. I'd like to believe that if I knew then what I know now about scours, I could have saved her. I'm sure I'll have another chance to find out, sooner or later...