Mild Steel Maximum Load

I’m going to construct a 6.0 meter long x 3.0 meter wide dismountable single story storeroom that will sit on three steel bearers on my asphalt car park. I have some 6 meter lengths of 3.2mm thick 100mm x 100mm Square Hollow Section Mild Steel Tubes (9.52 kg/m in weight) and I would like to use three of them as the floor bearers (they will be spaced at 1500mm). I will place small concrete blocks under the steel bearers to keep them off the ground by about 50mm. The remainder of the frame will be timber and the wall/roof will be lightweight steel siding/roofing.
Kind of similar to the following photo but wider and with one extra bearer through the middle (probably with no steel cross pieces [unless recommended] and supported with concrete blocks sitting on my asphalt car park).








1/ What is the maximum load that 3.2mm thick 100mm x 100mm Square Hollow Section Mild Steel Tubes could handle in this situation?

2/ What spacing (under the steel bearers) should I put my concrete blocks?

3/ Would the 3.2mm thick 100mm x 100mm Square Hollow Section Mild Steel Tubes be suitable if I decide to add a second story room on top in the future?

Thank you.