On Linear Power Supplies, i have seen two or three 1000uf capacitors are used parallely (just after the bridge rectifier) as smoothing caps instead of using one 3000uf capacitor. Has it been designed so purposelessly ? or no special reason (are 3 x 1000uf slimier to one 3000uf)?

 

might have been for size.

 

Or the builder had lots of 1000uF capacitors on hand...

 

In other words, there's no secret advantage to multiple caps in parallel, versus a single cap. The equivalent series resistance and heat dissipation would be very slightly better for multiple caps, so it's remotely possible the 1,000µF caps were a cheaper or easier solution than choosing a single cap that was properly rated for the job.

I expected from the title that this question was going to be about using different types and values of caps in parallel, such as a big electrolytic and a 0.1µF ceramic. This is about filtering different frequencies more effectively. Electrolytics are too slow for high frequency filtering.