I have a Trek Crossrip with Shimano Claris. Front mech is 3 x 8 with 50 / 38 / 30 teeth. I would like to fit a Shimano Alivio chain set with 48 / 36 / 26 teeth, would this be compatible ??
You can most likely fit a MTB triple crank - if the bottom bracket shell is threaded you can fit a Hollowtech bottom bracket that works with 68mm shell with spacers or 73mm wide shell without spacers and accommodates the axle length of the Alivio crank.
However, you'll run into compatibility issues because MTB triple drivetrains use a chainline of 50mm, whereas road triples use 45mm.
Basically if you fit a MTB triple crank the chainrings will be 5mm too far outboard relative to the cassette sprockets which will make cross-chaining from large chainring to large sprocket very bad, likely give you shifting problems and front derailleur chain-rub issues.