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Former WWE superstar Tammy Sytch -- considered the first "WWE Diva" -- has finally checked into rehab ... after being arrested for DUI three times in 2015.

Multiple sources connected to Sytch tell TMZ Sports ... the wrestling legend (aka Sunny) reached out to friends last month informing them she was seeking treatment to get sober once and for all.

Before she checked in, Tammy was telling friends she was hopeful that her stint in rehab would "offset" possible jail time in her DUI cases. Tammy is set to be sentenced in May.

Our friends at ProWrestlingSheet.com spoke with Tammy's sister who described the treatment center as a "retreat that focuses on the body and mind while using a holistic approach offering yoga, music therapy, art and holistic supplements to help the body heal."