I want to cook chicken legs, but will be out of house for 2.5 hours at what temp do I need to cook it at so it's not over or under cooked? I have just a basic oven, so I can't set a timer.
Operating an oven when no one is home is not a good idea, although some people would disagree with me.
If you can find an appropriate slow cooker recipe that you like, that would be your best best bet. Dark meat chicken is well suitable to braising, which is what slow cookers are best at.
I cannot give you an exact temperature, as most slow cookers or crock pots have only "high" and "low" as settings... the real issue is leaving the chicken in long enough, but not so long it turns to mush and gets that overcooked taste. Your three hour interval is certainly long enough to be safe.
The hardest part is finding a recipe suitable for your three hour time interval. Most slow cooker recipes are designed for around 8 hours, so you can leave them while at work, although I suspect they would actually work very well in 3 hours--you could certainly try them and googling them is easy. I actually suspect most 8 hour slow cooker recipes are badly overcooked, but haven't done experiments to find out.
Many more traditional chicken braising recipes are much shorter; again, they can probably be adapted to your slow cooker at three hours as dark meat holds up well to longer cooking.
By googling three hours chicken legs, I was able to find some promising recipes, including this one for very basic "barbequed" chicken legs. That could be adapted easily for almost any sauce you like.