There were two three person bikes at Presteigne last year and three and four person bikes are well known in the low countries, mainly for an adult and two or three children. Therefore I think they are legal, just so long as they are properly equipped or adapted for the people on board.
We increasingly tend to comply with EU law in these issues and they are clearly legal elsewhere in the EU.
We also have the "tag-along" bikes that are attached to the bicycle and carry one or two youngsters also pedalling, so clearly not a trailer but an articulated bicycle with up to three in board.
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