Do a google (top line of this page - within the web site) for rectangular slabs.
On one of these threads, look up the reference for the USBR manual for rectangular slab analysis (Moody tables). It is about 6 Meg (I think) but is a very useful reference - many proportions of slab, support conditions, loading, etc & gives support loads, & moments in both directions.
For a three to one ratio, most of the load will be resisted by cantilever action from the other long edge. There will be minimal support from the short edges except for a bit near the corners.
I usually look up the 'Moody' tables & (very quickly) find the max moment (either at the edge of the beam action span, or in the centre of the long wall for cantilever moment). This will give you the thickness of the section & the reinforcement req'd.
I then check out the min reinforcement (req'd by code) & try to arrange a dia to match the spacing of the main reinf.
I am usually very surprised on how great an area will be only min reinf.
I did a WTP recently (a series of rect tanks in one structure, for sea water desal - 120 m long, 26 m wide, & 12 m deep). Once you do the above calcs, most of the time the max moment is the cantilever moment (bottom of the long wall) & because of the cubic relationship for moment with depth, the moment decreases rapidly to min reinf.
After you do the calcs, compare the moments for 'fixed on three sides, free on fourth' with 'fixed on three sides, supported on fourth'. Then check what you require to provide the fourth support (eg a longitudinal beam), & compare the designs.
All of the above depends on the size of your project. For a small slab - what the heck! No need for the above details.