With 3 to buy for we have had to stick to a budget partly so we don't go bankrupt and partly to keep it fair. I tend to make lots of lists, especially at Christmas where they have stockings as well as main presents. Last year, we said £20 each for the stockings from Santa and £50 each, plus a few joint presents, such as stuff for their room or games to play together.
We tend to get them things during the year as well, such as a new bike at the start of the summer when they most needed it. If there was something they really wanted then I might spend a bit more, such as when I got them their kitchen but they are spoiled rotten by friends and family at BDays and Christmas so I certainly don't feel the need to spend any more as a general rule.