Hola Alvaro.
I have simple questions:
- Are all those sites are on the same server?
- Do you have sites separation? (ask hosting support if you are not sure)
- Did you installed any firewall on those sites? Real, a working one!
If yes (1 and 2), even if we or you clean one site, others (still "ill") may still infect it after a hours or day. Sorry, but it may work like that! I seen it.
In your case, it's better & faster to ask HOSTING support to recover sites from backup, for example, 5 days old. Then update all of them, scan and secure as much as possible. On all sites (J3-J6) TODAY install a free firewall component: https://htprotect.org/en/#htprotect - it has also has scanner, can clean sites and can protect sites. Use features from it.
We cannot fix or clean all your sites. For example, if even you can't log in to the site, how can we? That's why I mentioned about using backup. And I hope you know and understand that you're using Joomla 3.x sites on your own risk only! Yes, I know it's a painful truth.