This is a lot like my DP is with DD2, but it passes (he was like it a bit with DD1 too). Their relationship has many years to develop and change. I wouldn't stress about it unless he seems upset about how things are. This time round he also has the distraction of the older two who probably clamour for his attention too. Spreading himself across all three is harder than with one or two, so I'd be guessing the noisier more persistent ones grab his attention. He likely feels that since you've got things with DD3 under control he'll go where he's more needed.

At least, that's how it is here!