Shorter .com names (3 letters or combinations) are already taken up so you would be lucky if you could find .com 3 letter domains and 4 letters are also now becoming hard to find. But the answere to your question is yet only if its extension is know, no one cares about 3 lettered .in name or .ca names.
People will remember your domain name if it isn't complicated to write/remember, easy words, mainly 3-10 letters long, longer than that, forget about it.