Thanks for your replies 
Thanks especially Mike Peacock for the info on the bus from Aber. I could even get a cab from Bangor, if necessary.
For the record, I'll be B&Bing for 5 nights in Llanberis at the end of April and hoping for a shot at the Welsh Threes then, weather permitting
The plan is to do a steady 3am ascent up the Llanberis path for a 6am start.
Never attempted anything like this before. My longest previous walk was from the foot of the Watkin Path to Moel Eilio and down to Llanberis, via Yr Aran, South ridge & Snowdon. I'm familiar with Crib Goch and the Glyders, but not with the Carneddau - hence the need for GOOD weather. I'm also sensible enough to know when to walk away.
Fitness level is good - I have a 1:38 half-marathon time, although I do appreciate that this is long-distance HILL-walking.
I've done quite a bit of research already - with more to do - so hopefully, I won't become a MR statistic 
Thanks again.