Hi I have just bought a new Orange Micro Dark head and would like to know if it would power 2 or even 3 x 15 or 18 inch speakers I know they are good for a 4x12 but don't know how it would react to larger speakers any help would be great thanks
Yes but why? As long as you match the impedance, you can run 100x18" speakers and create a total wall of sound. It probably won't sound better or louder than a single V30 in a tuned cab though.
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
It'll work as long the speaker has the appropriate impedence - in this case, 8 or 16 ohms IIRC.

How 'good' that is all depends on the speaker driver, and enclosure, and if there's an 'r' in the month, and....