Oh, absolutely. I don’t think the trailer has much to say about any ship, though I will note that Mako is the only Krew member Korra is shown spending solo time with (ie. in the vine-covered building and in front of the building guarded by the White Lotus).
There’s no indication that Korra’s leaving alone on a three year journey, in any case. The “three years later” title card cuts immediately to Korra cutting her hair, which must be chronologically earlier than every other clip in the trailer that doesn’t involve Korra’s convalescence. If Korra’s out of it for three years, it’s more likely that it took a long time for her to heal than that she went on a journey for three years… but I think it’s actually more likely that Korra returned to Republic City at some point like she said she would and spent plenty of time with the Krew in the interim.
There’s also no reason to assume that Mako would end up in another relationship even if Korra was gone for that long. He got back with Asami in Book 2 for a number of reasons, none of which are relevant now — he was still clearly upset about his breakup with Korra and hence on the rebound as a way to get over it, and he was still in super non-confrontational mode (because he felt like he’d lost Korra and Bolin) and therefore basically give in to whatever Asami wanted. His last words to Korra during the breakup were “I will always love you,” which doesn’t really sound like the words of someone who’s going to be out looking for another relationship. =P
I do agree that they’d probably feel like strangers if Korra really was gone for three years… but that’s part of the reason why I don’t think it makes much sense to separate Korra from the Krew for that long. I can’t really see what purpose it would have to destroy all of the camaraderie that the Krew built in Book 3 just to have them start over again. =/
Going to agree with the asker here re: Mako (or anyone in the show, really) likely getting involved with other people over the course of 3 years.
Whether the show will allude to it or not (and it’s unlikely it would, despite having delved into other more mature subject matters), realistically, it would be more likely than not for a an unattached person—especially someone young and attractive—to get into at least one relationship over the course of three years. That’s a long time for a 20-something guy with relationship experience to be chaste! Unless Mako has become a celibate monk over the timeskip…
And I’d also like to point out that just because a guy “loves” a particular girl doesn’t mean he won’t go around having less meaningful relationships with other women to fill in the gaps. Mako might reserve a special place in his heart for Korra, but I’m sure as a normal human being he has room for other girls (including Asami) in there as well.