I am knackered - I have spent several hours today pulling in 3 x 16 mm² into 25 mm conduit. OSG promised me that it was possible and indeed it has been, but how difficult does it have to be?
Lube the cable, up the ladder, pull a bit, back to the beginning, lube the cable, back outside again, up the ladder, pull a bit. Down the ladder, move the ladder, up the ladder, pull a bit, down the ladder. Repeat. Repeat. Repeat.
Am I missing something?
Hahaha... rewired 100s' of old council flats', where the cooker runs' were the worst with old 7/044 VIR cables STUCK inside the conduit! BUT, pulling in 6mm singles, (ESPECIALLY when, back then, we'd sometimes been given SOLID 6mm, instead of stranded were another nightmare, haha),... glad I don't do that anymore BUT; answering your post... I always used lard etc, (don't tell anyone... sometimes fairy liquid, when desperate), but you must use someone to pull, while you're, (I know you know what to do...), feeding, the cables' in BUT BUT, haha, when the draw wire separated, haha! BUT , I hear you say... "what about the air gap?" WOT AIR, haha, GAP?
The other alternative is 2" x 2" metal trunking... I cannot remember, but must have at one point, used 32mm metal conduit, I can remember, actually, my foreman using a large section, around 5 foot long, of timber, with a hole at one end, used for bending metal conduit. I know I'd use the trunking, before using 32mm stuff. TOP tip; if you're running metal trunking and you've worked out you need to start off with 4" the always go one higher... use a 6" then drop down, at the first branch, to a smaller one!
Happy days', (on reflection).
Wot a life, a? 
All the best! Tom
