Which is the odd one out? 1, 5 or 3?

Come in number 5, your time is up.

1 and 3 are black-headed gulls in winter plumage. No. 5 is a common gull, Larus canus (which translates as 'dog gull').  Now 'Dog', used as an adjective, can be disparaging: it means something is not quite the real thing.  That is particularly apt here as this gull really isn't common at all.
Over winter, they tend to hang out with flocks of black-headed gulls and here in Fife, around an estimated 3% can be common gulls.

Their main distinguishing feature is their greenish legs.  Otherwise they look like smaller herring gulls.