I am 300 plus pounds and I smoked marijuana less than a gram about 18 days ago is it out of my system yet?

No. 30-30 days is required to clear the body of marijuana. It is stored in the body's fat cells and slowly clears.
Almost certainly yes. A single use of marijuana, no matter how heavy someone is, is unlikely to be detected after 2-3 days, and certainly should be negative after almost 3 weeks. Regular use of marijuana however, can remain detectable, particularly in people with a lot of fat (where thc metabolites are stored) for many weeks and even months.