How simple is the installation of a Netgear range extender?

It is a mesh extender so they do work pretty well in that sense. But most routers should cover a smaller 3 bedroom if properly located. You might just try centrally locating the router so it provides better coverage. They broadcast omnidirectional (spherical) so try to make sure the router is in the middle. 

What actual speeds do you get in your office? 

When you say intermittent, is your office connected to the 2.4ghz or 5ghz? I've found in most apartment/townhomes/condo's that 2.4ghz is pretty much useless other than slow IoT devices.