That depends upon how realistic weight limit is...Sounds like it's just the normal rpg way, not the "tactical immersion" inventory.
From the sounds of it you can carry any amount of weapons up to the weight limit bit have 3 *equipped * not a limit of carrying 3 weapons.
But I guess you may be right.
Even a single rocket launcher crammed into a backpack isn't realistic... Let alone a number of them.
I'd argue that such large weapons(rpgs, heavy machine guns, anti-materiel rifles) should be impossible to conceal and would have to be carried visibly attached to some belt, lock-in holster/whatever, and therefore they should either take up a weapon slot, or remain in the car trunk. Backpack not being an option.
But that's just my cup of tea. Gameplay wise I can see that some would like more liberty at realism's expense.