Robert Griffin III's Wife Blindsided By Divorce and Destroyed Over New GF

Robert Griffin III and his wife have been separated for months -- after she found out he was dating a new woman ... and sources connected to the situation tell us she's absolutely devastated.

Sources close to the estranged couple tell us RG3 first met Florida State track star Grete Sadeiko through Instagram back in January ... roughly 7 months after his wife, Rebecca Liddicoat, gave birth to their daughter.

We're told the social media relationship turned real ... and when Rebecca found out about her in April, she was pissed.

The two separated shortly after. They haven't lived together in months.

But get this ... one solid source tells us Rebecca was still working on trying to save the marriage as recently as this week ... and was blindsided on Tuesday by news that RG3 is "in the process of filing for divorce."

Now, with RG3 getting Grete's name tattooed on his arm, we're told Rebecca has given up on a possible reconciliation.

Bottom line ... this was no amicable split.