the 5th guy can take a weapon rack or dropped weapon instead of a carbine
so your choices are
5 carbines(base)
1x carbine 4x thompson
1 slot weapon and 4 thompsons
2+ slot weapons and no thompson
is what im comprehending here
But he specifically said 3 slots.
Your explanation only describes 2 slots with thompsons taking up one,.
If you so either the Thompsons override the slots, leaving only one place for an upgrade, or they somehow take up 2 slots leaving one left.
Either way, they have 3 slots and thus can hold 3 bars or zooks.