I just went through this. I got my 80K bonus at the end of the billing cycle where I completed my 3K in spending,. I got the 3K elite night almost immediately, and the 80K posted about 3 days after the end of the billing cycle. If you spent 3K, then I think you'll see it in a few days when it ends. If you didn't spent 3K yet, yes, you likely will miss it since it would be the next billing cycle.
If you spent 3K and it's been a few days past billing cycle, I'd definitely call Cust Services or hit up one of the reps on here for assistance. When I had a problem with my 15 elite night credits they got in resolved in literally 24 hours.
One thing I've noticed as well about Marriott is that you can book your room prior to having the points. Not sure if you've done that, but if you need to lock something in you can.