Glad to hear you didn't get hurt and that you solved your problem pretty inexpensively and easily.
I think it was just a coincidence that you used DOT 3 brake fluid and also suffered a hydraulic failure in the same system. As Tony mentioned, DOT 3 and DOT 4 are fully compatible and miscible, the only effective difference is the boiling point. Using DOT 3 in a system calling for DOT 4 would not cause any seal or other soft part failure, it would only allow the brake fluid to boil inside the caliper at a slightly lower temperature. In fact a lot of the available brake fluids are actually marked DOT 3 / DOT 4 meaning they can be used in either system (but the are really DOT 4 fluids due to the boiling point rating).
Also as Tony points out, DOT 3 and DOT 4 brake fluid are compatible but neither one is compatible with DOT 5. The other brake fluid that IS compatible with DOT 3 &4 is DOT 5.1. I know this makes absolutely no sense but the fact is that DOT 5 brake fluid is the odd- ball and does not 'play nicely' with any other brake fluid type.
So- these fluids will mix and vary according to the temperature at which each one boils (the higher the DOT number, the higher the boiling point):
DOT 3
DOT 4
DOT 5.1
They are all polyethylene glycol based, and a higher number can be substituted if a lower number is called for (example: system calls for DOT 3, that system can use DOT 3 or DOT4 or DOT 5.1 fluid) but not the other way- do not substitute a lower number than the system calls for.
DOT 5 brake fluid is silicone based and not compatible nor miscible with any other brake fluid. It can be used in <most> brake systems but the system must be fully flushed of the other type brake fluid before it is used. If the two types are mixed the resulting fluid will turn thick and clog the brake system very possibly making the brakes non- functional. The advantage of this type of brake fluid is that it will not absorb moisture (it is not hydroscopic), and it will not attach paint and plastics like the other fluid types. The downsides include it being slightly more compressible and so resulting in braking systems feeling 'softer', and the fact that it will absorb air making it interesting to bleed a system that uses this material.
Brian
I popped a rear brake line(don't ask) about a year ago. I replaced it and had a gallon of DOT3, compliments of my former employer. I thought the only difference in 3, 4, and 5 were the temperature ratings(wrong).
The other day as I was pulling into my driveway, the rear peddle bottomed out and had no effect. Although it might have been slid in under warranty, I was able to locate one off Ebay(the whole system, reservoir, hoses, cylinder) for $22 shipped from Florida.
It wouldn't be worth loading the bike on the truck and doing the 70 mile round trip for that, so I did it myself. $9.50 for a can of DOT4 and for under $35.00, I'm back in business.
I'd probably still be waiting, if I'd taken it to the dealer.
Anyway lesson learned 
