Based on the above 3 parameters lets now discuss parameter no.1- Identify the source of leakages
First of all do the ponding test with water so as to identify the source of leakages, once the source is identified then mark all the surfaces with a marker so that the source of leakages is identified properly.
Once that is done next you need to identify whether there is any structural cracking etc which has led to the leakages and if at all you see visible cracks more than 1mm in width then affix glass tape over the cracks and observe for at least 48 hours to see if the glass tape breaks, if it breaks then there is a serious concern which needs to be addressed ( this can be discussed if you are facing this problem) otherwise you can move to the last step.
The last step is attending to the leakages since you have mentioned that leakages are from the slab then we can use the following methods:
Cheap and easy - Make a thick cement slurry of proportion (1:1) and even spread over the slab either using a brush or a broom, this generally works in most of the cases. if not then try step no 2.
Bit costly and easy - Make a slurry by mixing 1 part cement and 1 part Sunanda Polyalk WP and apply minimum 2 coats, remember that if you are applying 1st coat in XX direction then apply 2nd coat in YY direction and note that you should leave 24 hours between each coats.( costing for material ranges from Rs 8 to 12 per sqft based on surface )
PU coating - If the above 2 treatments dont work then you can go for a 2 parts system PU coating which needs to be applied by skilled persons, there are various products available in the market which work but mind you these are costly systems. ( costing for material ranges from Rs 25 to 50 per sqft).
Note : Before application of the coating if there is profuse leakage then such portions need to be grouted so as to stop the profuse leakages and only then start with application of the coatings.
Hope that your question is addressed....Cheers