T3Cs and HACs do well in PvE (in your scenario Legion, Sacrilege/Zealot) , but in general not as good as a faction/pirate battleship or marauder . They are being used in situations where battleships are not ideal .
If you want a HAC for PvE , Ishtar is the perfect universal choice , but that’s not amarr , as for T3C is Tengu , but that’s caldari.
For faction / pirate cruisers , like I said , they are mostly used in PvP , there is where they shine mostly , unless there’s a Gila for example (that does a good job in a lot of PvE scenarios and also PvP) or an exploration Stratios.
With Myrm , you should have no problems doing level 3 missions , here’s an alpha friendly tested fit:
[Myrmidon, **Simulated Myrmidon Fitting]
Drone Damage Amplifier II
Drone Damage Amplifier II
Thermal Armor Hardener II
EM Armor Hardener II
Medium Armor Repairer II
Medium Armor Repairer II
10MN Monopropellant Enduring Afterburner
Large Compact Pb-Acid Cap Battery
Cap Recharger II
Drone Navigation Computer I
Cap Recharger II
200mm Prototype Gauss Gun
200mm Prototype Gauss Gun
200mm Prototype Gauss Gun
200mm Prototype Gauss Gun
200mm Prototype Gauss Gun
Medium Capacitor Control Circuit I
Medium Capacitor Control Circuit I
Medium Ancillary Current Router I
Hammerhead I x5
Antimatter Charge M x800
Use proper light and medium drones and hardeners .
If ISK is a problem for you , as alpha i suggest you do some abyssal sites , exploration outside highsec and some ninja gas huffing in wormholes , you’ll get more ISK doing that, you’ll get more ISK even ratting in lowsec in a destroyer like an Algos.