It depends on where the extra one is. I'm no fan of frogging, especially lace, if there's ANY other way to fix a problem. Yeah, you may be able to see it, but most of the time it won't show enough that even you would be able to find it later. Before you drive yourself crazy, make sure you don't just have a stitch stretched so tight that it looks like two loops on the needle, and make sure you haven't simply miscounted.
If there really is an extra stitch, lay out the lace and see if you can spot where the extra one crept in on you. If it's on an edge, you're in luck; just dispose of it by knitting two together, as you said. If it's in the lace section, and it just happened on the row before, knit up to the mistake, fix the row before (there are "Fixing Mistakes" videos on here if you can watch them), then go on with the row.
If you can't find it just by looking, start the row and see where the pattern stops matching. If you're going along "YO, knit two together, knit two together, yarn over...hey, that doesn't line up with the last row!" you'll know the problem is somewhere down the column from there. That's where to get rid of the stitch. If you can't find it even when you try to look down the work, definitely do knit three together rather than leave out the yarn over. Sometimes, when you're dealing with a whole lot of stitches, you never do find where it went off, but if you can get everything properly aligned again, it'll look fine.