7:52 AM PT -- T.I. and Tiny Harris tell TMZ ... "On the heels of positivity, negativity always rears its ugly head. This plaintiff has been threatening to file this lawsuit for THREE years. For THREE years, we have emphatically and categorically denied these allegations. For THREE years we have maintained our innocence and refused to pay these extortionate demands for things we didn't do. For THREE years, we've maintained the same position while the claims in this story have changed time and time again. Our position is clear... We are innocent of these fake claims, we will not be shaken down, and we look forward to our day in court."

T.I. and his wife Tiny Harris are accused of drugging a woman they met in a nightclub, and then taking her to their hotel for a threesome ... according to the lawsuit the woman's now filed against the couple.

The plaintiff, who's filed as a Jane Doe, says she was enlisted in the U.S. Air Force and stationed in L.A. in or around 2005 when she and a friend went to a party at Coolio's home and met a guy who went by "Caviar" ... who, they allege, worked for T.I. and Tiny.

In the lawsuit, obtained by TMZ, the plaintiff says T.I. and Tiny no-showed at the party, but "Caviar" told her they would be at an L.A. nightclub the next night, and that's when they all met ... in the VIP section.

The woman says she ordered 2 Amaretto sours on her own, before Tiny allegedly handed her another drink ... one from which she never saw Tiny sip, and which the woman also shared with her friend.

When T.I. decided to leave the club, the woman claims he asked her to ride with Tiny and him ... while her friend would ride with "Caviar," who said they were all going to the same place.

But, that's not how it played out, allegedly ... in the suit, the woman says 2 other women she didn't know got in an SUV with Tiny, T.I. and herself, and all 5 of them ended up in a hotel room -- but Tiny kicked out the other 2 women, because one was flirting too much with T.I.

Once it was just the 3 of them ... she alleges Tiny took her into the bathroom, where she took off both of their clothes and then a naked T.I. joined them and they all got in the shower.

Afterward, she claims she was starting to feel lightheaded and extremely dizzy, and T.I. told her to go to the bed. While he watched some porn, she alleges he handed her massage oil and demanded she give him a rub down. Eventually, Tiny joined them and allegedly pinned the woman on her stomach and began rubbing and grinding her naked body on the plaintiff.

While that was happening, she alleges T.I. penetrated her vagina with his toe, and she told him "no" as she tried to push him away. According to the suit, he got up and said he was going to get a condom -- but the plaintiff says she went to the bathroom to vomit. She claims T.I. mockingly laughed at her and said, "Are you alright? Looks like you in last place."

Shortly thereafter, she says she passed out until the next morning, when a security guard woke her up. She claims her "vagina was in serious pain" -- itching and burning -- but the security guard told she had to go, and escorted her out while T.I. and Tiny were still asleep in the bed.

She is suing the Harrises for sexual battery, assault, negligence and false imprisonment.

BTW, the plaintiff says she's filing the suit now due to California's Sexual Abuse and Cover Up Accountability Act ... which opened a year-long window for the revival of claims where the statute of limitations would have otherwise run.

A lawyer for T.I. and Tiny tells TMZ ... "Not only are the claims bogus, but they missed the deadline to file the lawsuit."