I just purchase my third Firespray! How would you fly them close together or separately?

You want to fly them so that one will always have its flanks covered by another.

Alternatively, you could fly them with one as a flanker and the other two as a unit.

Or fly them independently.

Just make sure to concentrate your fire. 'Tis better to kill one ship than to wound 3, even if you overkilled the one, as wounded ships can still bite back.

3 firespray are fun but in 100 point games you have nothing left for upgrades. I really dislike large based ships without EU.

Fly them very carefully.

yeah,
Concentrated fire is ideal but trying to hold formation in a standard game with large bases is difficult, so be prepared to split up.

While placing asteroids try to make large areas of open space if you can
You can space the asteroids in a vertical or horizontal line and make fly lanes!
or try to concentrate them in an area you avoid. if that makes sense!
The worst situation is a semi dense cluster in the center of the board...

Panic...