In cities Skylines, can two-lane one-way roads be upgraded to three-lane one-way highways?

Yes, as HorusKol says, you can simply upgrade them by using the "!" button (in amongst "straight", "curved" and "freeform").

However, when laying out the city, take special care to leave space around the roads you plan to upgrade, and don't attempt any close/tight intersections. The 3-lane highway is quite a bit wider than a 2-lane road, and this will have 1 of 3 effects when you upgrade:

• Demolishable things like ordinary buildings or power lines will light up in orange when you hover over the section, and will be demolished when you click. Make a pass afterward to fix any broken power lines or pedestrian paths (especially underpasses).
• Un-demolishable things like roads and buildings that you paid to build (e.g. fire stations) will glow red when you hover. You cannot upgrade until you relocate or manually demolish all of these.
• Sometimes, road intersections will not like the change, will glow red and prevent the upgrade. Usually, you can fire up the bulldozer, demolish the conflicting roads, do the upgrade, and then re-connect the other roads. Sometimes it just doesn't work.

Any road can be upgraded (or downgraded) to any other road - including highways - as long as you have the money and have the new road type unlocked

Bear in mind that any buildings on a non-highway road will lose their road connection, and any zoned tiles will also dezone, when you upgrade to a highway.