I'd heard that as well about 3rd births being more like the first birth, and my 3rd one was probably the most painful, BUT....
1st birth - ds1, 9lb12oz, 12 days overdue, induced in hospital, and had epidural. Was about 12 hours in total, about 8 hours in hospital (labour had actually started but they continued with induction ), so don't know how actually painful it was towards the end. Certainly 2nd stage (pushing) was longer than in subsequent deliveries. Had 2nd degree tear, stitched.
2nd birth - dd, 10lb, at home, VERY fast - contractions started 7am, dd born 10.10 am. Midwife arrived 9:40, 10cm already, half an hour later she popped out. Didn't push at all, just breathed her out, lovely birth! No pain relief, did have 2nd degree tear, but not stitched.
3rd birth - ds2, 10lb2oz, again at home. Started having gentle but painful contractions Sunday evening, but managed to sleep until about 5:30/6:00. Very painful but irregular contractions from then on, settled down about 9:30, mw came 11:30ish. It was VERY long, and felt like not much progress - after 3 hours had progressed to 6-7cm (from 4-5 when mw arrived). Once she broke my waters and got me to pretend to hulahoop (to get him down) things moved on, and it was VERY VERY painful. I gave birth standing up, and it was the worst at that stage, but of course the 1st one was disguised by the epidural. I was definitely the most tired - it had been SO long to get going, and I thought I was going to be sick it was so painful. But I didn't have any pain relief, and as with dd, had a 2nd degree tear but no stitches.