You're probably acquainted with the fact that not all antimalware applications can find them all, every time. Some have a larger and more up to date database than others. They will almost always have a fair bit of overlap, and also, even if up to date, will not share all the items database to database.
Knowing this it would seem that the risk should be reduced by adding more applications, to cover any shortfalls. There comes a point where the required investment in computer resource to have two, three or more applications scanning the same stuff outweighs any slight increased margin of protection offered by having them.
I have heard of cases where WD did actually detect something that some of the others did not.
I've also heard of many more cases where SAS or MBAM detected something that WD did not.
Another factor is: even if it is detected, can the application clean it?
From what I've read, Avast, SAS and MBAM all have the edge in this regard. Depends on the particular malware.
So even though with the setup you have, there is a small chance that something could slip through, I believe that there isn't much point in having WD active alongside Avast and SAS. Those two, (or Avast/MBAM) running resident, should prevent 99.? % of anything bad encountered.
I don't know what conflictions are likely with more than one AS running resident, it probably depends on each individual computer setup, but having more than one is likely to increase the risk.
Passive immunity, like SpywareBlaster, or a Hosts file, uses no resource, and simply blocks known bad sites/activex from loading. Too many immunity-type programs will slightly slow browsing. Maybe by a few microseconds for each item to load on a page. So, immunity=good. Programs offering immunizing include SpybotS&D, a hosts file (Google "mvps hosts"), and SpywareBlaster. Probably OK just having one of these active.