SFC3 is more fast paced as compared to OP, where PvP matches in OP can realistically last around 30 minutes or more. The ships in SFC3 are refittable, to add flexibility and make a ship more comfortable to each individual player.
The Dynaverse 3 is the same as Dynaverse 2, but unlike the D2, if a player engages in battle in the same hex as another on the D3, other players aren't drafted, but they can join in on the attack. Also, on the D3, you can right click on a hex and see who is in the hex, be it allied or friendly pilots or AI. Multiplayer skirmishes are the same when it comes to numbers, 6 players max.
SFC3 only allows you to have one ship at a time under your direct control, but you can approve 2 AI ships to fleet up with you, where as SFC2 and OP allow you to purchase 3 ships and fly them all the time.
SFC2 and OP however do engage you more in fleet actions, and can be quite enjoyable. In OP and SFC2, fleet cohesion is emphasized more than it is in SFC3, but SFC3 also requires strong coordination and fleet support in order to win a campaign.
One of the downfalls of SFC3 is the 4 playable races. Since SFC3 was rushed into production by Activision, Taldren wasn't able to finish the game code. There were originally supposed to be 9 playable races in SFC3, with a ton more stock ships and weapons/systems, and the stardate clock was supposed to be programmed so that certain ships and systems wouldn't be available until a certain date, just as OP, but unfortunately, that never happened.