That is sadly not the case. For the trinity stratagem to work, you need to have models firing with all three weapon type while being in range.
DS will put the Seraphims 9" away from their target. Their special stratagem will allow them to shoot as if it's the shooting phase. The bolt pistols are in range, the hand flamers have their range extended to 12" by the seraphim stratagem, but the inferno pistols only fire at 6". They're out the range : the holy trinity stratagem cannot be used.
Realistically, it's only going to be used by Seraphims one turn after their drop, or by BSSs or Dominions right after disembarking.
A Dominion squad of 6 Sisters armed with 4 meltas, one boltgun and a combi-flamer for the superior might also be a good unit for that stratagem. Sadly, it's not clear wether a single model firing with both profiles of a combi weapon can fill two slots regarding this stratagem. If it does, then 4 meltas and a combi-flamer will be enough for dominions.