I think the 8" can possible handle the 3 tents, but not with a restrictive duct setup, or fittings that go from 4" to three 3" ports.
I'm not seeing any 3 way 8" duct splitter fittings, and why I suggested just cutting up a plastic tote to use as one.
If anything, you might just hookup the 8" with whatever fittings you come up with, and if it isn't good enough, then install a smaller cheaper inline duct fan as an intake booster for each tent, to effectively make it into an active system, instead of passively trying to pull through 3 tents. Or use one intake fan going to the 3 tents, just like the exhaust with another 3 way split.