I live in a one bathroon home. I had a brand new toilet installed. It worked great for about three weeks. Now, the water drains all the way out when flushed, the almosts runs over and it makes a sucking sound

You have a blockage. Try snaking out with a closet auger. If that doesn't work, the toilet needs to be pulled off and check in bottom of toilet for blockage and/or snake out toilet waste drain.

Thank you for your response but what is an auger? Also, is there any magic potion I can pour down the toilet until I can afford a plumber?

A closet auger is a tool used to snake through the toilet itself... see image. A closet auger can be purchased at any home improvement store for about $30.00 or so... ;)

Chemicals are not going to help here!

Mark

I agree with Mark on both points but if you have a Menards around you you can get it for $16. Who did the install?