The final 3 mods are flexible. There's 3 main ways to build them:

Viral+Rad: 165Elec+Blaze+165Cold/ChillingReload - Requires Toxin bonus. No flex slots. Riven replaces mod with same stats.

Viral+HM: HM+165Cold/ChillingReload+Flex:
Riven, if you have one.
Faction - Highest DPS increase, but only vs 1 faction.
165Cold/ChillingReload - Whichever you didn't pick for your mandatory mod (1 Cold mod is required).
PTactical Pump - Low sustained DPS increase, but makes Reload conveniently short.
6060Tox - Low burst DPS increase, medium sustained DPS increase.

Corrosive: 165Elec+2Flex mods:
Riven, if you have one.
6060/90Tox - Mandatory if you don't have a Toxin bonus or if you want ChillingReload/165Cold in the final slot.
Faction - Highest DPS increase, but only vs 1 faction. Build has no DOT damage tho, so DPS difference isn't as big as HunterMunitions builds.
165Cold - Slightly less DPS than a PFaction mod, but works vs all factions.
Blaze - Medium DPS increase and extra Armor penetration for enemies that take multiple shots to kill.
Chilling Reload - Low DPS increase, but adds convenience through faster reloads.
PTactical Pump - Better than [Chilling Reload] vs enemies weak to Corrosive, but worse vs everything else.

Additionally, there's a Magnetic build that's good vs Sisters and Hounds, but bad vs most other (unshielded)enemies: 165Cold+165Elec+Faction(Corpus)