Let's put it this way; I have set up many wireless networks and it would take me a couple hours to get your network working. You have no experience, so I expect it would take you longer, or you may never get it to work. Three floors is a difficult setup for wireless.
With the HomePlug adapters your whole setup will be 15 minutes (and ten minutes of that will be just setting up the router). To network with HomPlug you just take the adapters out of the box, plug the Ethernet cable into your PCs and router and plug into the electrical outlet. You are networked.
With wireless you have to install a bunch of software and drivers on your PC, figure out all the settings to match the wireless AP in the router, figure out where to put the router to get good wireless coverage for your whole home, move PCs around until they get a good signal from the router, etc. It's a lot of work. If you think it's worth saving $50 for all that work, go for wireless. Lot's of people have trouble and never get it to work and return the wireless equipment. Wireless LAN equipment is the most returned item at the electronics stores for that reason.
The speed of your DSL is 3Mbps maximum. HomePlug 1.0 data rate is 5 to 8 Mbps, so your DSL is still slower than your home network would be, so no you will not be slowed down by using powerline networking.