Hi Tatopitta,
First of all, why are these time fields text types? They should be time types, no? To sum time, do you want to have the same format? Hours:Min? If so, it will require some business logic (code) since there is no default function for summing times. It should be easy to add this though. Just add an OnAfterChange handler that updates the total time based on the others. Probably less than 10 lines of code.
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra