Have them kiss him while he gropes them.

"Hey Cana," Natsu grinned. "How about you kiss me awhile I feel you up?"

"What!? Why the hell would you ask that of me?" Cana demanded.

"Sorry, sorry," he smirked, before looking at Lucy as Cana fumed, red in the face, looking about ready to use her magic cards on him. "How about you do it Lucy?"

"Sure thing Natsu!" Lucy grinned, striding right over, and planting a kiss on his lips as he reached up, pulled her top open, and proceeded to start groping her.

Cana stopped dead, staring, and felt... Jealous of Lucy? She stared as the two continued making out, Natsu giving Lucy's boobs a squeeze. Then he broke the kiss and grinned.

"...can I...?" Cana asked, blushing and looking away.

"Can you what?" Natsu asked.

"...do what I just declined to do?" Cana asserted with a blush.

"Sure," he grinned, and she smiled before moving her arm from her topless boobs and walked over, cupping his face and leaning in to kiss him, waving her melons enticingly, and he reached up to cup them.