I'm having my old bike repainted and am considering adding braze-ons. I'm adding two for water bottles, but I have questions on others: (1) Can old Campy down-tube shifters be converted from clamp-on to braze-on? (2) Can I get a braze-on front derailleur for a wide-range triple (24-40-54)? and (3) Any other braze-on suggestions? Thanks.

I suggest you check Johs Putnam's list here:
https://www.phred.org/~josh/build/brazeons.html

As for your questions:

(1) Can old Campy down-tube shifters be converted from clamp-on to braze-on?

Yes, if you find the proper braze ons. Or would it be the occasion to upgrade to indexed shifters? Or to move them to bar-end shifters and use the downtube stops for the Shimano screw-on barrel adjusters?

Add an eyelet near the headtube, through which the cable housing will go, so it won't rub the paint.

(2) Can I get a braze-on front derailleur for a wide-range triple (24-40-54)?

Yes, but many don't recommend it. Rivendell states clearly on their website that they have seen frames breaking at that point and I have read about some manufacturers strenghtening their fraime (i.e. thicker walls) to account for the braze on. I have also read about people with broken frame. Besides, once you position the braze on, there is no coming back. What if you finally prefer 24-38-48 and need to slide the derailleur down by 5-6 mm?

(3)Other ideas?
If your frame is large, can you position two bottles along the downtube, plus one on the seattube and one under the downtube?
Pump peg. My preferred pump is the Topeak Road Morph, with bracket installed under a bottle
Brakes. I am assuming you have a road bike. Are the brakes good? Are you considering a replacement to cantilever or v-brakes? If so, canti posts might be good.

Braze-ons can be had from several sources on the web. I've modified several bikes that way, just doing it on an old Motobecane. I generally add cable stops for shifters and brakes, downtube shifter bosses, water bottle bosses, sometimes I move brake bridge and add canti brake bosses, pump boss, front derrauleur mount boss and rack boss barrel. Some I silver braze, some brass braze. The cleaning and painting afterward is lot harder that the brazing part, but it's worth it to me. The source I usually use is: https://store.airbomb.com/ba/asp/cc.105/ab/items.htm.

You might look at current practice for STI types which is to braze cable stops on the side of the
head tube or the down tube adjacent to the head tube. Original practice was to have the cable
stops 6-10" down the down tube in the same place as downtube shifters were mounted and to
have an adjustable screw there also but this practice was abandoned in favor of the head tube
or near head tube mounts with no or simplified adjusters. There are water bottle mounts that
only need a hole in the tube into which the mount is placed and it locks itself into place. No brazing
needed. Steve