In reply to stoker:
When you say "double rope" do you mean "half rope" or "twin rope"? there is an important difference.
when we "double rope" for UK trad we climb on "Half ropes" whereby only 1 rope is clipped into each piece of gear. each individual rope is capable of holding a sizable leader fall as often only 1 rope will be loaded in a fall.
"Twin ropes" are something very different. they are largely designed for big multi-pitch continental style water ice climbing. they are thinner and lighter than "Half ropes" and are both clipped to each piece of gear. they are not designed to hold a sizable leader fall independantly. the reason for two ultra thin ropes is it allowes twice the absail distance therefore saving allot of time on approaches/descents. it also makes carrying gear fairer than climbing on a single.
my understanding is that "simul-climbing" is what the americans do on big walls sometimes, where a pair will climb together with the full length of rope out between them (see the well known video of Uli steck and Alex Honald speed climbing in Yosemite). i wouldnt advise this as a 3.
Moving together "alpine stlye" is much better known in the UK where both climbers take coils so that you only have about 12-15m of rope between you. this is done predominantly on "Alpine stlye" routes which tend to be mostly scramble terrane with some short technical steps (and are also often ridges). it is perfectly acceptable (if not common) to do this as a 3 (or more). the guys at the front and back take coils as per usual and the third ties in in the middle so that you maintaine about 12m between each person. the best way to tie in in the middle is an alpine butterfly or a double/rethreaded overhand knot as opposed to a figure of 8.
it is perfectly fine to climb technical stuff on 1 (not doubled up) "half rope" in this configuration, but make sure your rope isnt a "twin rope".
if you get to a bit which looks like you want a propper belay, then the guy in front takes off their coils, giving themselfs an extra 15m or so of rope and leads as if pitching. both seconds then simlul-climb with the middle man unclipping and re-clipping the rope to the gear as normal. the leader can then take in coils again and resume moving together mith minimal rope faff.