Getting a Thule roof rack for my 54 Astra, anyone know if i will be able to fit three bikes on top?

I've had four on the top of my Zafira (same fittings and width I think) so three should be no problem. You might need to top and tail or turn the stems at 90 degrees to avoid bashing the bars together though.

Cheers for that, 3 carriers it is then.

Wife had a 54 plate Astra and 3 bikes fitted on fine. The middle rack needs to face rearwards though, unless you slacken the stems and twist the bars through 90deg.