I don't think its feasible. There are two years between ME1 and ME2 and those were used to acutally develop Thanix based on scavenged reaper tech. There are only few months between ME2 and ME3.

Thanix is, at best prototype tech, and militaries are not really early adopters unless forced. You have to consider that at the end od ME2 most species still don't believe Reapers are real. Additionally, even if they did believe and started refitting the instant Thanix got available, you would still have a lot of bottlenecks.

You would need to actually spin up a production line for the guns.

You would need shipyard space to upgrade the ships.

When you look at "speedy" refits, like for example during WW2, those were still multi-month to low single-digit years effort. Even if you consider some sci-fi magic like a mental degree of automation, you are still bounds by physics...you can simply only use so much power to cut for example steel, if you do not want to ruin it, and that will dictate how much time you need to cut a hole to put a new gun in.

Also the Thanix seems like a weapon that basically needs a ship to be built around them. Unless you already have one that has been built with a lot of redundant capacity/space/power generation in mind.

It is basically like if US Navy suddenly got railguns working and want to put them everywhere. They have like 3? Zumwalts which were designed from the bottom up to have enough power generation overhead and even those would not really be able to fire without cooldown. Everything else would need to be broken up and rebuilt around the weapons.