re 3V - I picked up about 75 from Tesco yesterday - 7 failed though so I only ended up with 68.

re The Range - The Range allows a maximum of £6k per month, but to do that you would need to get nearly all £100 vouchers, which are not that common.
I've done about £5k so far through RPPMC but to be honest having to drive to Reading or Aldershot to collect the cards, then drive home and load the card at 3 vouchers per day, then chas back out to your bank.
IMO more hassle than it's worth, as that £5k only earnt me 6,250 Avios
You also have the hassle of dealing with RPPMC phone staff when half the vouchers you buy don't work, and then have to answer 100 questions about why you're buying so many vouchers in the first place.

In comparison spending £5k on 3V anywhere else apart from Tesco will get me 12,500 - I have spotted well over 100 in one boots store, nabbed 40 from a Morrisons recently, and that's without even looking for them.