Merton is inside Tommy’s head when Lori kisses him.
It’s been established that Tommy can feel whatever Merton does in his head and vice versa, so this truly, canonically, is a three-way kiss where each one person is kissing the other two. After the kiss happens Lori is left breathless, Merton is blissed-out, and Tommy fully transforms.
Lori has said before that Merton was a great kisser, and indicated she likes his intellect and insight. She also likes Tommy, his strength, his appearance, and their common interests. She says that combined they make “one great guy” implying she likes aspects of both but neither one is enough for her by himself.
Tommy actually wolfed out during this kiss. By season 3 he’d gotten over his ‘wolf-out whenever he kisses someone’ phase. He’d made out with Lori before for extended periods of time and not reacted this way. So if Tommy is feeling this kiss from both Lori and Merton, then the added factor has to be what wolfed him out.