Given the host of benefits these grenades have over regular frags, and the relatively high damage output, it doesn't really make sense - balancing wise - for players to be able to carry 3 of them.
Their throw arc and reduced timer make it much easier to nail targets than a regular Frag grenade - and on top of that you get 3 of them, with each grenade being capable of inflicting more than half your health damage!
Even with Flak Jacket's capacity reduction, their low weight would still make them a formidable option that in no way would feel underpowered