Q1. Butterworth - 1st order.
Q2. Diagram 2 is not necessarily 3rd order since there is a feedback path to the RC network shown in each stage. There is no statement of the specific response for each stage. To be a third order filter one would anticipate that each stage has no actual feedback to the RC network and hence each (stage) is a simple single pole filter. I can't comment on the merits of using the Diagram 2 option in relation to a known Butterworth format other than to state that the Butterworth is intended to give a maximally flat response. If maximal flatness is the requirement then one would probably choose Butterworth.
Q3. Yes all in one stage is certainly possible for 3-pole.
Check this link ...http://www.beis.de/Elektronik/Filter/Act3PoleLP.html
Q2. Diagram 2 is not necessarily 3rd order since there is a feedback path to the RC network shown in each stage. There is no statement of the specific response for each stage. To be a third order filter one would anticipate that each stage has no actual feedback to the RC network and hence each (stage) is a simple single pole filter. I can't comment on the merits of using the Diagram 2 option in relation to a known Butterworth format other than to state that the Butterworth is intended to give a maximally flat response. If maximal flatness is the requirement then one would probably choose Butterworth.
Q3. Yes all in one stage is certainly possible for 3-pole.
Check this link ...http://www.beis.de/Elektronik/Filter/Act3PoleLP.html