I'm afraid you're blowed up.
The #3 main cap is also the end thrust main. Failure to have a shop properly fit the new cap to the block WILL result in crankshaft journal failure.
I had the exact same thing happen to me. I had a '54 235 block bored with new cam bearings and freeze plugs installed. When I got the engine back, I noticed a crack in the #1 main cap. I planned on using the #1 cap from another engine, but it would bind the crank up. I tried to have it line-honed, but no shop in MS had a line-bore bar that could do a 235. Since a 235 has "tapered Mains" each journal is a different diameter, unlike 99.9% of the engines currently being rebuilt. So I had to throw the block away and rebuild a '60 235 instead.
I'll look thru my pile of junk and see if I can dig out a good #3 cap. I do hope Im wrong, but only a machine shop can measure the fitment and tell for sure. BTW, does your engine use shims? That complicates the issue even more. Some say that shim motors don't need line-boring, and some say its more critical. Again, consult your local quality machine shop. Which yours doesn't sound like a good shop, any half-decent shop magnafluxes the parts for cracks before working on it.
