Well for one thing if you've only got one can then you've only got half the flux. Peterson #3 is a two part flux with a mixture A and mixture B.
Peterson #1 is made for brazing steel and other materials that are easy to flux.
Peterson #2 is made for cast iron and malleable iron at medium and high heat. I've been forced to try this lately since I can't find any of the #3. It seems to work almost as well as the #3 on castings that haven't been soaked in oil. I've not had a chance to try it on the really difficult castings yet.
Peterson #3 is made for cast iron and malleable iron and low and medium heat. In other words it works at a lower temperature than the other fluxes allowing less heat to be put into the work piece. My former employer wouldn't use any other kind of flux on cast iron. I do know it works very well even on dirty castings.
If anybody ever finds any #3 and wants to sell it let me know. I need some more. I've got one set of cans left at the moment. But I have discovered that the #2 works almost as well on clean castings so I'm working at saving the #3 for the hard jobs.
From the label there only appears to be a few differences in #2 and #3. One that I can remember off hand is the addition of limestone to the #3. One of these days I'm going to do some experimenting and see if I can just add a few things to the #2 and make it work as well as the #3.
NO. 3 SPECIAL for brazing cast and malleable iron at low and medium heat.
Instructions on the back of the can read as follows:
Take from the can, Two ( 2 ) envelopes only of each color, and mix the contents together throughly in a small flux container or can. Small flux container may be left open when not in use. ---For low heat brazing: Bevel by grinding and heat the casting to a red heat on edges before applying flux. Apply flux with a wiping stroke, using the side of the rod so that you can completely wet and coat the metal with the melted flux as far as bronze will flow.
CLEANS FASTER------- BONDS STRONGER
Bronze can be applied from a dull red heat up to the molten point of the cast iron. Rusty, greasy, burned castings easily brazed with No.
3 Special. Cracks may be veed out with high e xcess oxygen flame or electric arc.
I tried to open it again today to look inside, but did not open it. Kinda rusted shut. Might open it later if I am feeling lucky.
Actually now that I think about it the first cans of this stuff I saw where that way. That was when I first started working at that shop and the next set we got came in separate cans as have all the others I've seen.