Office is THE easiest 3 star, mad easy (once you take the glass out beforehand). Also got this game recently to mop up the 1000G which took a few days. Spent hours going through the time trials 3 starring them all to realise you only need 50 stars. So after smashing my head against the wall I could just 1 and 2 star the rest fairly easily for the cheevs.
Deff recommend the time trials before the speedruns as there's a bunch of parts the same so you can use the same quick roots which a number of youtube video's take longer routes in these parts.
Last level speed run was a bit of a pain.