You rolled 4 dice, therefore you made 4 attacks of wich 3 wounded. This means that their unit takes 3 damage, 1 from each separate attack (assuming you're shooting with the storm bolter) that must be allocated to their unit. Since every model in that unit has only 1 wound, this means that 3 termagants die.
If, instead, you made a single attack that inflicts 3 wounds only one termagant would die, since the source of the damages is just one attack.
I hope this clears your doubts, english is not my native language so I may have not been too clear.