"In December 2011 I put four of these on my patio to hold posts for a covered work area. They are simple, solid and cost a lot less than the big chain DiY stores want for similar fittings. 18 months on I just took the structure down and they are still perfectly usable.
Just bear in mind these are 75mm x 75mm and a lot of 3" square fence posts on the market work out a bit smaller than that. The posts I bought had to be packed out to fit because the adjusting bolts wouldn't clamp them tight enough."