[CentOS] Getting to Mate or Xfce

My wife bought a new Thinkpad, on which we had the shop install CentOS 7 (I *think* it's 7.3.) Alas!, I didn't think to specify NOT Gnome, in thunder. So of course I got it.

I've tried several times to force myself to accommodate Gnome3, always with no joy. How do I get Mate or Xfce instead?

Related problem: when I become root and tell it "yum update," it figures out a list of umpteen hundred rpms to update, but then runs through attempts at mirrors and keeps failing till it runs out. It failed to connect to my wireless router (or even to find it, afaict). So I plugged it into the router with an ethernet cable. No joy.