Why not just locate a LARGE frame (26 inch electroforged frame) VARSITY or CONTINENTAL and then convert it to 3 speed configuration. It would be fairly simple to do. Easiest way would be to also have a decent Schwinn Three Speed in whatever frame size, women's-step through or men's version.
ABOUT THE ONLY THING YOU WILL BE MISSING WITH THE LARGE TenSpeed FRAME IS THE Welded on tab where the Chainguard bolts above the hanger set(bottom bracket). Heck, that is simple to fabricate a bracket that goes around the frame tube.....heck you can go to Home depot/Lowes and buy a $0.89 electrical conduit hanger bracket.....called "CONDUIT HANGER"for 3/4" Steel EMT and Rigid Conduit (UPC code 031857491109).....This 3/4" steel strapping bracket will fit snugly around the One Inch (25.4mm) Schwinn frame tubing.......you might then need a small piece of metal, about 1/3 inch to 3/4 inch in length that you can fabricate from almost anything IF you want to connect this CONDUIT HANGER to THE FACTORY LOCATION ON ANY SCHWINN CHAINGUARD OF THE 1950's/1960's/1970's.
SCHWINN made it extremely EASY to identify and determine WHAT size Weinmann "Schwinn-Approved" Caliper Brake Assembly that YOU WOULD NEED BASED ON THE SIZE OF THE FACTORY SCHWINN WHEEL THAT YOU WOULD USE IN THAT FRAME beginning in 1969, according to GLENN's COMPLETE BICYCLE MANUAL (c) 1973 approx 340 pages 8 1/2 x 11" softbound.....more than a half million copies printed and sold in the years 1973 - 1976 and beyond. From Chapter 10, -Overhauling Brakes- on page 307 it explains that beginning in 1969 that Schwinn had a visible stamped code on the face of the outer brake arm to indicate the type of wheel on which the caliper brake assembly is to be used.....etc
You've got something like LS 2.4 for the Varsity/Suburban that has 27 inch (630mm) wheels, and you have something like LS 2.8 for the Collegiate/Breeze/Speedster etc with 26 inch (597mm) wheels............. .....................L (that first letter) tells you that caliper goes with the Schwinn LIGHTWEIGHT MODELS' wheels.
S (that second letter) tells you it is SIDE PULL...................................The numbers tell you this------------the TWO tells you that the whole inches without added fraction portion is TWO INCHES from center of mounting bolt to center of brake pad mounting slot-----+++++ then the .8 --------------each "dot whatever" (tenth) is equal to 3/32 inches...................so the "point eight" is 3/32 times EIGHT.........................................thus two inches plus 24/32 or 2 24/32 iches or 2 3/4 inches.................
Schwinn didn't change anything really from the CALIPERS that were seen on 1968 models, as I seem to recall the 1968 Collegiate brake calipers say something like Weinmann 810 or something like that but they are the exact same as those on a 1971 which says LS 2.8
Schwinn simply did this to make it easier for their dealer shops and maybe the factory too, to be able to easily determine what goes with what.
Okay, now if you see a FIRST LETTER that starts with M, that goes with MIDDLEWEIGHT SCHWINN and if you see a first letter B, that goes with BALLOON TIRED WHEELS.
The Second Letter indicates whether it is S (side pull) or C (center pull) , that probably will be easy to see without having that, but if you had the caliper completely disassembled in many pieces, perhaps you might could get it wrong if you were drunk maybe.
I highly recommend that you obtain a $4 free shipping copy from the bay, Many giant booksellers with large ebay operation have these old books and you can always find a USED Copy for about $4 to $5 total with free shipping. YOU NEED THAT BOOK ON YOUR SHELF, IF YOU OWN ANY BICYCLE THAT WAS MADE BETWEEN 1935 and 1982. Glenn's Complete Bicycle Manual by Clarence W. Coles and Harold T. Glenn (c) 1973 340 pages covers everything from coaster brake models to ten speeds from every manufacturer.....................if it was made between WWII and the mid 1970's, you can bet there is helpful information on how to maintain and adjust, disassemble and repair!
As for SCHWINN not making the exact "nameplate" 3 SPEED bicycles in the gigantic frame size that the Varsity/Continental were offered with during some years in the 1970's..................................HECK, you can buy the REPRO Decals that say Schwinn Speedster, Schwinn Traveler, Schwinn Breeze, or whatever the heck Schwinn model came with 3 speed...................................and Bingo then after that, you've got the SCHWINN size that fits you................With the DECALS and shiny new paint, IT WILL BE A SCHWINN 3 SPEED, that the factory custom built for you.........just don't tell others that in this case the "Factory" was in your backyard. As the Nike ad says, 'JUST DO IT!'. You'll have fun.
There is a 26 inch VARSITY FRAME on the bay with zero bids at opening bid of $16 and $36 to ship it to my region. There is no stem, no seat tube, no calipers, but THERE IS KICKSTAND, HEADBADGE, FRONT FORK and the 1977 era Varsity graphics and green paint look okay. For less than $60, you can have the gigantic frame at your doorstep..............to build a 3 speed that will fit you. Yes, you'll be overpaying for it because you can buy complete Varsities for from about free to $50 but it may take driving around and hunting awhile to find the needed gigantic frame sized Varsity.