Sudden change in computers accesses within same workgroup

[France42]

Dear all, could you please help me for the following:
we have three computers in a private network, in a same workgroup. They are all 3 running under Windows 10. There is no Homegroup, only a Workgroup. Until recently, each computer could see any of the other two and access shared files and printers.
Since a few days, computer 3 cannot see computer 1 anymore (in file explorer). All the other exchanges work ( 3 sees 2, 1 sees 2 and 3, 2 sees 1 and 3).
I have made no voluntary change. The only thing which might have happened is a Windows update.

I have made sure that:
- shared computers and printers are still set as such in the various computers. Nothing has changed, in fact;
- the various needed services (like DNS client, etc) are running. They are actually set to run in automatic mode;

I can ping computer 1 from computer 3. It works. Still computer 3 does not see computer 1 in its file explorer.
I also temporarily disabled the antivirus (Avira) and Windows defender on computer 3 to see whether 1 would then be able to see 3. No, it does no change anything.

At this point, I don’t know how to resolve the problem.
Any suggestion is welcome. Thank you very much.

France.

 

[France42]

I forgot to say that computer 3 can print stuff using the printer linked to computer 1 (it is the the default printer for all 3 computers of the network). The problem narrows down to the fact that computer 3 does not see computer 1 in its file explorer, under "Network" (but it sees computer 2).
Thanks fr your help.