1) 3b Rav Ashi adds on to the words of Rebbe Nasan that a mishmar and a half can also be called two mishmaros. The question is that if we would not come on to the extra 2 hours of when the other melachim woke up, then since we are only dealing with a period of 6 hours, why would we be forced to understand that Rebbe Nasan held 4+2 and that there are 3 periods of 4 hours, maybe they imply 3+3 and there are really 4 periods of 3 hours, like Rebbe?
2) 3b What is the Hashkafa understanding in "Lchu U'Pushtu Yedeichem B'Gdud" in other words why was that the right thing to do?
3) 3b Also, according to Rashi in d"h V'nimlachin that explains that they asked Sanhedrin so that they should daven for their success, it would have made much more sense to ask the Urim V'Tumim first, in case it would say that they would not be successful?
1) Sorry, I don't understand. Rebbi Nasan finds what he considers an irrefutable source for 3 4-hour Mishmaros in the word Tichonah. Why should he change his opinion on that matter?
2) David was advising them to reclaim areas which the gentiles had taken from the Jews earlier, but the Jews were afraid to fight over. Alternatively, he was referring to areas of Israel that had never been conquered. Another possibility is that he allowed them to kill the nations that were deserving of death in any case (such as those who killed David's father and brothers), like the Ramban in Vayishlach in his discussion of Shechem.
3) The rule is that we never ask for Hash-m's more direct intervention until all other means at our disposal have been tried. It is the last on the line, since we don't want to be impudent and bother the Ribono Shel Olam over every question.
I hope this answers your questions.
Be well, and best regards,
-Mordecai