I presume you are referring to the Jordan Pass.
This gives discounted/free entry to multiple attractions across Jordan, but its real benefit is that the cost of the Tourist Visa to visit Jordan is waived if you stay a minimum of 3 nights/4 days.
If you are staying only 3 days then you will not be able to avail yourself of the free Visa. If you attempt to use the 'free' Visa for a shorter stay you will be able to entry the country without paying, but on exit they will confirm how long you stayed in the country, and if it was less than 3 nights then you will be forced to pay the visa fee at that point.
Without the free visa it's unlikely that purchasing the Jordan Pass will be beneficial for you, but it will depend on exactly which attractions you plan to visit.