It is important to understand the different type of threats to your PC. For example you mention Avast and Malwarebytes, these are two totally different products that look for different things.
Avast mainly looks for viruses so is an anti-virus product.
Malwarebytes looks for spyware / malware so in an anti-malware product.
You should only have ONE anti-virus product installed (as they can conflict with each other and slow the PC down).
But you can have 2 or 3 anti-malware / spyware products installed.
MSE claims to look for viruses AND spyware, and generally gets good reviews. Under the covers MSE includes / works with Windows Defender (also from Microsoft).
So you could stay with Avast for virus checking, or remove Avast and replace it with MSE for virus checking.
You could KEEP Malwarebytes and also install MSE, so you would have 2 products looking for spyware / malware.
Note MSE is working all the time looking for spyware, whereas the free Malwarebytes product is an on-demand scanner so only works if you run it manually.
Not used Spyware doctor so cannot comment on that.