Just a couple of thoughts, Rose.

As you're doing 3 as opposed to the pyramid style in the book are you also doing a warm up? The first two sets done a la BFL are warm up sets, with light weight, to losen the muscles.

The biggest difference you would notice, IMHO, would be if you'd been doing the 3 x 15 for a while and were continuing on with it - the body responds to change (and being shocked), which is why the HIIT works so well for those of us who've been doing hours and hours of moderate cardio.

WTG on finishing up W1; here's to W2

Nat