Please analyze Claudio´s behaviour in Act 3 of "Much Ado About Nothing"?Especially his relationships/conflicts with other characters.

Claudio makes an important decision in Act III.  He
decides that there is concrete proof of Hero's infidelity.  He does this based upon
events that happen offstage and only reported to the audience in Act III, Scene iii by
Borachio, Don John's henchman.

The only scene in which
Claudio actually appears in Act III is Scene ii, in which he first  (along with Don
Pedro and Leonato) teases Benedick about cleaning himself up and shaving, presumably for
Beatrice.  They continue the good natured, friendly banter that they have shared up to
this point in the play. By the way, this joking and bantering about will take an ugly
turn in Act V.  But in Act III, all still appears to be fun and games amongst the three
comrades.

Then Don John enters.  He is Don Pedro's half
brother and has sworn to ruin things for Claudio.  His main motivation for his dastardly
acts?  He calls himself  "a plain dealing villain."  So basically, he just likes to stir
up trouble.  Already, Don John has tried to convince Claudio that Don Pedro (seen
dancing with Hero, his face hidden by a mask in Act II) was making a play for Hero
behind Claudio's back.  And though Claudio gullibly took the bait then, but was proven
wrong, he shows himself to be still susceptible to Don John's accusations here in Act
III.  Don John basically says that he has proof of Hero's infidelity, and will take Don
Pedro and Claudio to stand beneath her window so that they can see for
themselves.

Though sometimes staged as a dumb show in
performance, the scene in which Claudio and Don Pedro watch the pretend Hero (played
unwittingly by Hero's serving woman Margaret) engage in a tryst with Borachio is not
included in the play.  So, the audience is left to wonder why Claudio would be so
foolish as to believe Don John, who has already proved himself suspect with his first
accusations of Hero.  Why do you think he believes Don John?  Shakespeare doesn't give
us any definite motivation, and seems to have purposely left this very important event
-- the witnessing of "Hero's" infidelity out of the play.   A couple of things about
Claudio, however, can be said.   He appears to be easily made jealous, and he's pretty
gullible to boot.