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I read your query and after analysing all facts and co-relation of symptoms it is very unlikely to have STD
in your case after 3 years. The blisters above the lips may be viral or some other skin erruptins because you have passed the incubation period
Now I am going to answer your queries point wise-
1- Sir for contamination only momentary contact with infected person is sufficient irrespective of seconds.
2- In your case STD is unlikely because for infectious diseases there is definite time interval between cotact and development of symptoms and this is called as INCUBATION PERIOD. In your case blisters appeared after 3 years which is a much larger interval than incubation period so blisters in your case are unlikely for STD.
3- There are various test to confirm STD, but the simplest one is VDRL
test. This is a blood test
so you can do this test to clear the doubt.