Using selected speed and flaps to initiate initial approach, then with gear down and selecting flap 3 I’ll then go back to managed speed to reduce workload (this is what I’ve seen in videos). However, flaps 3 and 4 don’t ever reduce speed to Vapp speed as shown in MCDU, so unless I go back to selected speed I’ll come in too fast. What am I doing wrong? Is this something to do with the timing of when you activate approach phase on the MCDU - if activate and then go selected (for flaps 1&2), can you then not get back into approach phase on MCDU to coordinate speeds and automatically manage speeds for flaps 3&4? 

I had this problem too, the only (temporary) way I know to solve this is to turn off my autothrust, set the thrust to idle and set the speedbrakes. And I think it's better to use manual thrust for the approach.

My solution (or this might be normal procedure?) is to not select the activate approach phase on MCDU if intending to use selected speeds through flaps 1-2. I suspect if you activate approach phase on MCDU before selecting ‘selected’ speeds through flaps 1-2 that the ‘selected’ speeds disarm the prior activation phase on MCDU, which you can’t reactivate later. So, what works is this: selected speeds and flaps 1-2; gear down; activate approach phase MCDU; then command speeds to follow flaps 3-full automatically.