1- a chain tensioner typically mounts to the same hole that a rear derailleur threads into. Just put your hanger back on the bike.
2- If you use the middle ring of your current crankset, you need to make sure the chain is shortened to accommodate for the smaller ring. Also, that middle ring is usually ramped to aid in easy shifting. This means it is not designed to hold onto the chain, and with single speed you want to hold onto the chain.


Personally? I would question if single speed with a chain tensioner is worth the time and result. You lose out on the direct feel of single speed when you have a chain tensioner. And I would buy a narrow-wide chainring to hold the chain better then remove the other rings up front.



If you are going to just have a tensioner and keep your current triple crankset, why do the conversion at all? Just ride the bike as is in whatever single gear you want and call it good. Just dont shift and you will have the same setup you are spending time and money on achieving.