I am doing some heavy commuting these days, trying to keep the miles off the car. I am riding a Windsor Tourist with two water bottle cages. I would like to add a third like many of the touring folks have under the down tube. But how do they mount the cage? Do they drill the frame and add threaded inserts or something?

Yes, a bicycle builder will probably need to braze some bosses in there.

An easier route would be an Aqua-Rack. I've been using one for years and love it. It will add two more bottles on the back of your seat (getting a drink is as easy as scratching your butt).

You could use something like this "universal cage mount". Clamp it around the tube and screw your cage to it instead of the frame itself.

https://www.amazon.com/Topeak-TCM01-C.../dp/B003SOROKE

This thread offers some choices as well. I think the aquarack or something similar is mentioned.

https://www.bikeforums.net/showthread...r-Bottle-Mount

Zip-ties.

There are bikes that already have a third (or more) set of braze-ons. They're mostly touring and rando bikes.

back in the days of standard tube sizes, water bottle cages came with tube clamps to hold them on. I have used hose clamps too. Smaller frame sizes often don't have enough room under the down tube. You might consider a handlebar water bottle cage

I've got my second cage mount where the OP wants their third one. Lemme just say that is a less than ideal place to keep a bottle with anything you plan on ingesting...

Assuming you mean because of road debris, I had the same problem riding my mountain bike. I solved it with the Nalgene ATB bottle. The cover doesn't seal water tight, but it keeps the teat clean. Works for me.

I'd just throw a third bottle in my panniers or use a handlebar bag with a camelbak style bladder. Or, if I absolutely had to have the third cage, I'd zip tie it on. No, I wouldn't add braze-ons myself, I'd take it to a framebuilder if I was going that route.

Under the downtube is a pretty dirty place for a waterbottle. A lot of the touring folks use that cage to hold their fuel bottle. I recommend you use one of those behind-the-saddle bottle cages. It'll give you two extra bottles.

Why not use a hydration pack instead? One hydration pack can hold almost as much as three water bottles. But if you really want to use water bottles, allow me to suggest the Minoura water bottle mount for handlebars. It's much more convenient and easier to reach.

This is so... The other thing that gets bolted on down there are frame pumps. One trick to keeping the pumps in working order is to put a condom over the end. Would this work for a water bottle? XXL condom??

my Touring Bike has 2 on top of the down tube , 1 beneath, usually the stove petrol bottle down there .

though there are flip cap covered water bottles that keep the drink cap clean..
Sigg for example makes one, .. , Nalgene, as above, another..
I also have a seat tube bottle cage..

Oh and there is the 2 fish velcro strap on cage,
and adapters to put a bottle or 2 on the handlebars..

I followed that link and noticed they had one that mounted to the stem as well.

Unless you have a really long commute (2 hours or more) in hot weather, two large water bottles should be more than enough. I've seen as large as 1000 ml bottles (Nashbar sells one for $6), but there are probably even larger ones. Your frame could limit the size bottles you carry. You can also tank up a bit before riding, which would help, and definitely keep fully hydrated throught the day and in the evening after riding. Also, why not just carry a spare bottle of spring water in your bag and stop if you really need it?

Re: under the downtube: I have a cage mounted there, I just used clamps. It rarely gets used unless I am on a long trip where the availability of refills is uncertain. Accessibility is not an issue because I just rotate an empty down below when I am stopped somewhere (Trying to retrieve use and return a waterbottle to the underside of my downtube is theoretically possible, yet not something that I'd personally attempt while underway)

That said, the underside of the downtube is in fact a dirty place to mount a bottle. Also, in recent decades better mount options have become available (See all suggestions above).

I actually do have a 2L. hydration pack. It has about 600" of storage,too. I guess I never gave this a lot of thought for the bike. Maybe I should give it try this weekend.

It would make sense. The original Camelbak was invented by a cyclist for cyclists.