From very beginning to end, for my four boys:
1st was 8 hours (5 days early),
2nd was 4 hours (6 days late),
3rd was 2 hours 55 minutes (1 day early)
So they got quicker each time, but the babies got heavier each time - 1st was 7lb 9oz, 2nd was 8lb 4oz and 3rd was 8lb 12oz.
(4th took over the three hours though, but he was also lighter than 2nd and 3rd at 7lb 9oz and he was one day late, meaning he started school a year later than he would have had he been born the day before, which is what I wanted.)
Ds3 and ds4 were both planned home births, it is a good job they were as I wouldn't have made it to hospital for either of them!
So, for the majority, the heavier they were the less time labour took. But that isn't indicative of what will happen with anyone else!
All in all I had it easy with all four labours and births and they all came close to their due dates, so no, third babies do not have to be harder labours or arrive late, as others have already witnessed to on here.