The best way to teach a 3 year old to pick up his toy room every night before going to bed, is by helping him do it as you show him how it is done. Then you make it a habit of reminding him that, the room should be left clean and award him every time he does it.
He probably has to many toys to deal with. Next time he is napping put most of them away, then put just a few of his favorites in the toy box. It is much easier for you and him/her to deal with picking up three or four. If he understands well you can try to explain the reasons for having less toys and tell them you are practicing taking out, playing and picking up, until you both get better at it.
You probably won't. But once he sees that won't help he'll start to understand. Take the trash can in and explain that if he don't put them up then he will have to throw them away. Worked with all 3 of mine. Given I did throw some of my Sons away once never had a problem again and their 11,9,8.