I remember those desires at that age too - now that my son is nearing age 2, I look back amazed at all the various transitions he made in schedules - whether that was sleeping, eating, playing, anything. I put so much effort into worrying, desiring & sometimes insisting that he sleep (which he rarely did until 15 mths - seriously), or eat a certain amount.
There are great books to glean some helpful hints on establishing routines (listed below), which is very important. But other than that, you can not make your kid sleep when & how long you want him to (at least I was never able to accomplish that!!). Definitely put him in his crib or wherever you want him to sleep at certain times of the day; definitely create your daily schedule so that there is consistent nap times established for him (meaning, don't schedule errands to be done when he typically naps). 3 months is the very beginning of when you can start to sleep train, but he's too young to be expecting that long of a sleep session, at any time of day.
Consider yourself BLESSED that you are getting a 7-hour stretch out of him, that is so great!
In my opinion, the best thing to do right now is take a few days & really pay attention to the specific times of the day that your son looks like he needs to sleep; jot that down somewhere. Once you get a specific idea of what HIS schedule usually is, then you can start molding YOUR schedule around that. Certainly you can put your son in his crib anytime you WANT him to nap, but you'll have a lot of crying, a lot of frustration, etc.
Sleep can be a consuming thing in this time of your relationship with your child - again, looking back, as much as I needed the sleep (and still do!!) I wish I had paid more attention to his signals & know that these times really are so short. My kid's naps usually lasted 20 minutes - once he hit 7 or 8 mths, they started to stretch out. But even though he goes to nap at a certain time of day every day, it's never consistent - sometimes an hour, sometimes 45 minutes, sometimes 3 hours! You just have to go with the flow (in my opinion) & try to enjoy this time you have with him. I know many will disagree with that concept & they have to, because they work, because they need a tighter schedule, etc. But I just did not have any luck with trying to force my schedule over his. You have to do what works for YOUR family.
Book recommendations:
"Healthy Sleep Habits, Happy Child" by Marc Weissbluth
"Solve Your Child's Sleep Problems: New, Revised, and Expanded Edition" by Richard Ferber
"The Happiest Baby on the Block: The New Way to Calm Crying and Help Your Newborn Baby Sleep Longer" by Harvey Karp