Do you 'try' and get them 'into' anything...or just take their lead?
Example - 3.6 yr DS old not really into anything (not interested in building, colouring, crafts, vehicles, sport) except a big talker, bit of cooking (10 min limit attention span), bit of TV after nursery in mornings only just starting to play imaginative play (v limited involving a scenario, not with any toys just 'You be the baddie, I'm the goodie')...
DD 2.7 has not fixed onto anything but likes a range of a few different games and things and I go with it. She does go through phases of doing a lot of one thing for a day or two sometimes but not usually. We did start her on trampolining for tots at the local sports centre after she started jumping on the beds and her old cot mattress to divert her from this less desirable activity! She loves that.
We have a friend who asked DH what our DD was "into", what was "her thing" and he looked at her blankly. Apparently she thought toddlers develop a speciality which she reckoned was drawing with her DS and was massively encouraging it. I really don't think pre-school/toddlers need to have an obvious favourite activity. I think your DS sounds normal!
If you think it is his attention span, that he spends little time on each activity or doesn't seem to respond enthusiastically, then it may be that he needs you to play with him. We visited another friend with a DS the same age as yours and she spent little time with him (not saying you do as I don't know ) as a result he spent less than a few minutes doing anything. He had no guidance with his play and so just drifted about before actually being naughty to get his mum's attention. 3.6 is young for a child to be expected just to amuse themselves for hours.