I wouldn't say you were feeding too much, but I would look at what you are feeding.  If you are concerned about their condition in any way, then you may need to make changes.  Why are they on grass nuts as well as a general mix along with grazing?  I haven't ever used grass nuts before, however a friend has used them to help supplement a lack of good grazing and to help condition horses, however they did make the horses a bit fizzy.  If you are using a good general mix you also shouldn't need to add vitamins on top of it as it should contain all they need.  You could change the grass nuts for sugar beet pulp and maybe give them a bit of a fibre based feed such as a good quality chaff.  To be honest you don't want to put too much weight on your 3 year old as the joints are still developing and growing, if over weight she would be at risk of DJD/OCD.  If they have a good condition score and what you are giving them is working I wouldn't change it too much.