From my local PC, that I will call "Machine A", I launch RD Tabs to remote desktop to "Machine B"
From "Machine B", I need to remote desktop to "Machine C"
"Machine C" is not accessible from "Machine A" directly.

Is it possible to use RD Tabs, remote desktop to "Machine B" and somehow remote desktop to "Machine C" and it be another tab on RD Tabs?

Before it suggested, I do not to open RD Tabs twice.
I current do something like that...open RD Tabs on "Machine A" to get into "Machine B", then launch mstsc to remote into "Machine C"
My desire is to have one copy of RD Tabs open and have multiple tabs.

I think I am asking for the moon, but I thought it would not hurt to ask.

That's not possible with RD Tabs and I'm not sure it would even be possible to implement in a way that's not exceedingly complicated. You might want to look into a port-forwarding solution. You could have Machine B port forward port 3389 from Machine C.