I've just purchased three 30w 6" light bars.
Question: Is there a valid reason why I can't use a single relay to power up all three units?

no 1 relay will power those 3 no bother mate its only a 7.5 amp draw

Cool. Might as well build my own rig ... can't seem to find a 3 or 4 light harness on the sphere.

The lights (90w) should be good enough for regular enhancement ... If not, I'll hang a 16" bar below the mounting rail and more than double the output. That'll have the truck moderately front end weaponised!

Ended up going for the Halo's currently being rebadged by Flatdog. If we're to believe the hype they're a cut above some of the others and sport the 10w Cree LED. We shall see.

easy enough to make your own loom up mate more fun too don't forget the pics!

Do you need a relay for these light bars

For anyone interested in the math:

I = P / U = 30 Watts / 12 Volts = 2.5 Amps

Well they arrived. There's three of these ... first impressions are they look reasonably well made. They come with corresponding plugged fly leads so I now have the spec to build my wiring loom. Got to get jiggy with fabricating an adaptor plate to fit my light rail.Bild entfernt (keine Rechte)

hey they look nice what plug is on the end of those fly leads

They're Deutsch DT's pretty nice plugs and sockets.
http://www.polevolt.co.uk/acatalog/Deutsch_Connectors.html

thought they were nicely made and water proff did they send you the male end for ur loom or do u have to buy them seperate

Yes ... they come with both male and female plug n socket.

cool so u just stick them on the end of ur loom jobs a good one