Hi Dennis,
You want the PHX-45, not the HV45.
Yes, you do need the BEC: that's what will power your timer/sequencer.
I didn't even think the HV runs all the way down to 3S: I always found a 12V lead-acid battery kind of marginal when powering it up just for the purposes of powering it up for the Castle Link.
The standard non-HV will work up to 5 cells, too. Castle advise against the BEC use on 5S, but that's because they are thinking of multiple servo loads (> 1 A) and our timer/sequencers are literally a few 10s of milli-Amps.
Hope that helps,
Dean