Sometimes 1 of my 3 toilets does not flush all the way. Eventually it will go down by itself. There is a gargling noise in the sink in the same bathroom when I try to flush or plunge it. Also when I take a shower again in the same bathroom you hear the gargling in toilet and/or sink. Is it bad venting? This bathroom was added on to the 35 year old house a few years ago. Is it bad venting or my septic tank needs cleaning.

I'm not a professional in any way - but I had similar problems and after some internet research found out that this "burping" can be caused by poor ventilation indeed. Check your plumbing vent pipes for obstruction, it could be something as simple as a bird trying to build a nest in there.
How to Clean Plumbing Vent Pipes | Home Guides | SF Gate

Hi Mjames

You and Katw88 are onto something with the vents, for sure! Here, I suspect that the sink drain/vent is clogged. By your description, I would have you look under the sink and see if you have a mechanical vent (called an air admittance valve or AAV) on the drain pipe. Here, the AAV is usually something that is threaded and can be removed (see image). If you find a removable AAV remove it and go to a local home improvement store and get a replacement AAV and some teflon tape and install the new AAV. If you have something that looks similar but is not removable see if you can post a picture here.

If you do not find an AAV under the cabinet then it sounds like the SINK DRAIN may be clogged up a bit. Here, I suspect that the sink drain and vent act to WET VENT the other plumbing fixtures in the bathroom, and that means that a clogged drain would effectively act as a blocked vent for your toilet/shower...;) Snake the sink drain in this case and it should improve things.

Mark