If you already have 2 good COM radios then you definitely do not want a GNC355, unless for some strange reason your audio panel is capable of 3 COM and you feel some weird need for 3 COM.
You say you have both ADS-B Out and ADS-B In. However, you don't say how you are doing it. If it is via a GTX345 transponder, then you should be able to see TIS-B (traffic) and FIS-B (weather) on your G3x. In that case, you can rule out the GNX375 since aside from IFR GPS navigation it doesn't do anything you aren't already doing. However, if it's via a Stratus ESGi or SkyBeacon/TailBeacon then you're probably getting FIS-B/TIS-B on your tablet rather than the G3x. If it's a Lynx NGT-9000 then you might getting FIS-B/TIS-B on its screen rather than the G3x. A GNX375 might be worth the expense since it would allow you to see all that info on the G3x rather than yet another device, which would especially be nice since traffic would show up as a growing yellow dot on the artificial horizon and/or synthetic vision instead of having to look down at an inset or switch to the traffic circle page.
So if you rule out the GNC355 and the GNX375, that leaves the GPS175 if all you want to do is add IFR GPS navigation. However, you might feel the need for a 2nd NAV radio. Some people do, some don't. It is certainly possible to train for IFR without it, though. If you do want a 2nd NAV radio, then you'll want to look at a GTN650xi or GTN750xi. However, going that route would mean removing your remote COM radio. And if you don't already have a GTX345 transponder then a GTN won't give you traffic and weather on your G3x like a GNX375 would.