Hi there,

first of all: cobs belong to the slow growing breeds and if she is only 3, she still has a lot of growing into herself to do. At the moment her bones will still be quite soft and she might very well not even have reached her final height yet. Don't expect her to grow another hand or anything but possibly an inch or two. So, if I was you, I'd give her another 18 to 24 months to mature properly before putting any weight on her back at all. When she has matured fully, she'll be able to take up to 20% of her own body weight. If you weigh 15stone (95 kilos) that means she needs to be about 475 kilos, which is fairly realistic for a 14.2 - 14.3h heavy weight cob.  Mine is 14h and weighs 450. But like I said: let her mature fully first! In the meantime, I would start doing all the ground work and by all means, when she is four break her to drive first and get her to pull light stuff, then at about 4.5 have her backed by a lightweight adult, and build her up to weight gradually. You should be absolutely fine. All the best!