A place to ask bicycle repair questions, and for bike shop monkeys to share advanced non commercial wrenching resources (no YouTube self promotion). This is only for repair related topics.
If this is on the front chain ring I would check your limit screws on your front derailleur to make sure it can actually swing all the way to the position for the third chainring.
Other possible causes, the pawls in the shifter getting gunked up. You could try cleaning it out with WD40 or alcohol. The put back in some oil like tri flow.
Final possibility the cable/housing could be shot and needs to be replaced.
Can't do it rn, but will check the limit screws when I can.
I'm guessing I'll have to disassemble it for that.
I'll check the cabels when I can.
The limit screws should be easily accessible. Usually they just take a small screw driver or Allen key to adjust and they typically have a small H and L next to it. You should adjust the H screw.
Do you know how to setup and preload the derailleur to get the proper indexing?
I'm not sure, but since 1 and 2 is fine, could it be indexing? I should've shown in the video that when switching 1-2 you have to apply constant pressure on the lever until it clicks. 2-3 feels like it snaps way earlier than it should, and there is no feedback force from the lever after the 'snap', like it doesn't pull the cable at all.
So I can't see what you posted with the link. My browser blocks tracking embedded in imgur links like this. I don't have any way of knowing what 1, 2, and 3 are in your instance. Better terminology would be inner outer and mid, or big, middle, and small ring. I don't blame you for not knowing the terminology or whatnot. I totally understand what it feels like on the other side of the coin.
Triple derailleurs are almost an art to get them shifting really well. It will likely require using an inline micro barrel adjuster spliced somewhere in the cable housing. On all FD's it is not intuitively obvious, but the height is absolutely critical. The outer cage should clear contacting the teeth of the outer ring by 1-2mm.
It is difficult to get the initial cable tension right for a FD, especially a triple. Explaining it in depth is not very effective here. I recommend watching a reputable youtuber for this if possible. It sounds like you probably do not have the proper tension on the cable for indexing. Some people use 3rd hand tools to preload the cable.
Did you fix this? It's hella easy - I had the same thing and watched a YouTube video, I think it was; flush the gunked up lube with WD-40 > re-lube.