Your router's firewall is the first and most significant line of defense against the Internet. You're fine. Your main vulnerability will be what you do with those computers (visit sketchy sites, download malware, etc). You're not invulnerable, but you're probably not hiding $10,000+ data, so accept the risk.
To exploit the workgroup feature specifically, the intruder would need direct access to one of your two computers or ethernet cables, or a backdoor onto one of them.
Even then, if the computers are protected with different account names and passwords, things like shared folders (unless you set them public) are shielded from an intruder on the other computer.
----------------- Below here was my first answer, where I made OP's question more complicated than it really was. -----------------
Based on what [I thought] you said of your connection: Internet > (possibly a router, 3g internet, or other direct connection) > PC > Router > Other computers.
If so, the only way to invade the "other computers" from the Internet would be to first invade the PC. With elevated privileges on the PC, one could use it as a platform to begin poking at the other two (by configuring routing on the PC or simply by executing attacks from a remote connection to it).
In this case, even if the two computers are as insecure as Windows 95, their security is equal to the security of your main PC (like building a big stone wall around a rickety shack).
Like the stone wall to rickety shack analogy, though, if that stone wall is breached, the hypothetical Windows 95 boxes are as vulnerable as shacks.
This presumes they do not reach out to the Internet, which would introduce a whole new level of insecurity.