Mounting Bearer at ground level

I'm building a small deck at my front door and have a concrete slab the width of the entrance but it only extends out about 1200mm from the door. I am going to have the deck come out about 3000mm from the front doors and so I need to put a few bearers extending out about 1800mm from the end of the slab. I plan to put steel C section as joists directly on top of the slab and the bearers.
At the moment the slab is about 80-90mm above ground level and ideally I'd love to be able to just dig out a shallow footing and throw in 3 150x50 bearers and use some quickset to keep them in place. I'm sure this is probably the dodgy way of doing things but probably the easiest. I'm wondering if there may be a more suitable way that is just as easy.
I'm trying to get away with the least amount of digging.

edit** I was just looking thru the bunnings catalog and saw a 20kg bag of readymix concrete for $4.40.
I'm wondering how many bags to the cubic metre. The area I need to cover is 1800 x 3000 x 100 which gives me about .54 cubic of concrete metres I need, with a bit of steel in there it might be easier as I do have access to a mixer and plenty of timber for form work.

Any idea's?

Cheers

From memory, a bag of concrete only does about 0.01m3, so you'd need 54 bags.

I got 0.8m3 delivered in a minimix for about $200 when I did the slab for my shed, and it was suggested, that for a slab that size (2200 x 3100 x 100), I wouldn't need any reo.

The truck was able to back right up to where I wanted it poured and it took me about an hour from when the truck turned up to finish it, very much worth it, in my opinion.

If you don't mind converting to and from Imperial, this:

http://www.quikrete.com/Calculator/Main.asp

can provide a quantity estimate. Your 100mm thickness is substantially 4 inches, and your area of 1.8m x 3m is about 58 sqft. So you'd need 33 - 80-lb bags, or 60 - 44-lb bags (20kg). Bunnings' manufacturer may have an equivalent website.

Joe