It is normal for boys to do that and as well as explaining when to and when not to do it it is also a great time to show them how to care for it. Keeping him (willie) clean is important and showing them the right way to do it may save you a trip to the doctors if willie gets an infection.

I have 2 boys (4 and 2).  It is absolutely normal for them to express themselves this way. You will find that they stop doing it when they are about 75!

This is normal behavior. I think the best you can do is to teach him that it's not OK to do in public.

Also, ignoring an unwanted behavior sometimes goes a long way. When he does this, perhaps you can try and ignore him? Don't smile, don't make eye contact.

I used to to casual work in a respite home for kids with brain injury. The standard practice there when a child was masterbating  was to ask the child to go to their room to do it in private. It's important that he understands that it is ok to touch himself but in private. Make sure he understands that he is not being sent to his room because he is naughty (you don;t want him getting a hangup about it), its just that his bedroom is the place it is ok to masterbate.

My boy has his hands down there quite a bit. I think its normal. Its just about exploring and learning. It has always been the 'toy' he hasn't had to share....

I think its just a thing boys do.  My 4 1/2 yo boy has just started doing this again about a week ago.  Every time I look at him he seems to be doing it (he doesn't do it when we are out, thank goodness).  The first time he started doing this was just after he was toilet trained but I suppose grew out of it.  I assumed it was because he wasn't wearing a nappy anymore and it probably felt different.  All I do now is everytime I see him playing with himself I tell him that its not acceptable to do as he is only 4 him to go and wash his hands. I figure its just a stage he is going through he will simply grow out if it or he will get sick of going to wash his hands and think he'll think about it twice before he playes with himself again.  At least I will know his hands are clean.

Touching is of course totally normal.  My suggestion is that at 3, he is old enough for you to explain that it is absolutely ok to investigate and to play with himself, but it is a private sort of thing and you would like him to save it for a private place.  Other people don;t like to watch him.  If you feel his is doing so much that he is missing out on other things going on around him - just check to make sure that he isn't under too much pressure - sometimes even the daily routines, particularly when there are other older children around can become a little too hectic for a fragile 3 year old to cope with.

My son does the same.  I say to him that if he wants to play like that, it is something he is to do in the privacy of his own room and he is to wash his hands.  He seems to always have his hands down his pants.  I don't make a big deal of it, I remind him to take his hands out of his pants and wash his hands.  I sometimes sit him down and remind him of the way he should behave.  My daughter used to love to have a feel, only during nappy changes.  She used to be so rough.  I used to say to her "soft" and "gentle".  She would wash her hands after I'd finished the nappy change.  She has no interest anymore. 

It must be a boy thing.

At the end of the day, you should not discourage him wanting to discover; instead encourage appropriate behaviour in regards to when and where.