You can’t use three Thunderbolt 3 ports to drive three displays. You can accomplish this by connecting two Thunderbolt 3 to HDMI adapters/cables and one HDMI to HDMI/VGA/DVI adapter via the HDMI port.
A couple of things to consider when connecting monitors:
Use native signals rather than convert. Thunderbolt 3 is also DisplayPort, so it’s better to go ...
I think you're getting into trouble by trying to run a monitor at 75 Hz. Either lower the resolution or the refresh rate. Try 800x600@60. Once that's stable, step it up a notch until you know the limits.
I would also be susipscious of the quality of your HDMI to VGA adapter/cable. Does your monitor have an HDMI ...
According to Apple
Up to three displays: Two displays with 4096-by-2304 resolution at
60Hz connected via Thunderbolt 3 plus one display with 4096-by-2160
resolution at 60Hz connected via HDMI 2.0 [...]
So it is important to not exceed the maximum resolution supported.
Your 1080p Monitors seem to meet that spec so you will need to look at your ...