I know exactly what you are talking about!! I have a 25 month old son and we have been having a rough time with him in the last few months. There are some days when it seems like he whines and cries all day long and it drives me insane!!!! You have some good advice here and I'll just share a few things that are helpful for us.
Keeping a consistent schedule helps to avoid a lot of tantrums before they start. With my son, I can't let him get too hungry or tired - either of those happen and it is the perfect recipe for a meltdown. He needs to eat very soon after I get him up in the morning and I cannot be late with lunch or naptime. It may not necessarily be this way with your daughter, I also have a 3 1/2 year old son, and he doesn't usually have trouble when he gets hungry, but he still needs the consistent routine. I know this can be difficult with a 3 month old, but it is possible. If you are not already doing this and would like some tips, I will be happy to share with you. It was a work in progress from birth but by the time each baby was 3 to 4 months old, we were pretty well established in a routine, with gradual changes as they grow and develop.
I always try to keep my boys informed, in advance, to changes in activity. 5 minute warning before we leave the park, or need to pick up toys, etc. Sometimes one of them would still have a fit, but it has been getting much better lately.
I try to never give in to the whining, crying or tantrums. First I made sure my son understood what I meant when I said "no whining" (I demonstrate a bit of what he is doing, not to mock him in any way, just so he knows the behavior that I am telling him not to do). And I have to tell him frequently, "you don't get anything for whining/crying." If he wants his milk, he needs to ask politely with a "happy voice" and I have him repeat after me (according to his language skills). If he is throwing a fit over something he cannot have, I give him an opportunity to stop, and if he is unable to, he gets 5 minutes alone in his crib. There he can get all of his screaming out of his system, and I get a little buffer with the walls. Sometimes he needs a series of these!
I also try to pay attention to his particular "fetishes." One day when we were at the park, he would whine and cry any time another kid got within 5 feet of him. I have learned he has a rigid sense of "personal space" (and I can sympathize, I am very much the same way). So the next time we went to the park, I talked to him before hand and told him there would be other kids around, but he was not to do any fussing if other kids were playing near him. It helped!
I am pretty certain my son is working on his second molars. Some days are worse than others, but it gives me hope that once those teeth finally come through, he might be a little more pleasant! But I know it's not all about that.
One thing that helps me, I go in to their room at night before I go to bed and check on them. One look at those adorable sleeping "babies" and one little touch of their soft, warm cheeks, and all the trouble of the day melts away!