I told my party this:

Don't turn on ANY buffs before they see a Flash go off. This includes, but is not limited to: Raging Strikes, Quelling Strikes, Divine Seal.

The way it works is when you run in, Twintania runs to you since you are the first person it sees. Not the dragons however - they can also target the nearest unit to them. For example, someone standing on the hill on the far left side might get targeted by the left-most dragon too.

When I've got all 3 of them on me, a single Flash should do it. Never really had a problem with the initial pull since.