I bought and built the house in Falkreath, but I can't buy land for the other two areas. I know about having to do quests in order to buy them, but the stewards there don't have an option for the quests. Can anyone help me out? If you can tell me the quest names that would be helpful.

Yep same problem here. I finished Waking Nightmare, But the Jarl just says thanks but does not give me the quest to kill the giant,

I don't know about Morthol, but in Dawnstar you have to generically ask for work and he will say to kill a giant in a radiant location (I had Blizzard's Rest). Took me a while to figure that out, too.

For Morthol you need to do the Laid to Rest quest. Inquire with the Inn keeper about the burned out house. She will tell you to speak with the Jarl.

For Dawnstar you need to do the Nightmare quest first. Then when you are ~ level 22 talk to the Jarl and he will want you to kill a Giant. Alternatively, do the Civil War quest line up to the point where you recapture the Pale (Fort Dunstad). The new Jarl will let you buy land if you have already done the Nightmare quest.

i didn't do the nightmare quest. but sided with the Empire and got the title of the land while they have nightmares.

I'm stuck at the same point. I did Waking Nightmares and got the Jarl's thanks. As it happens my character was lvl 21 at the time so the Jarl didn't ask me to kill giants. However, I asked at the inn and got the usual "jarl's men left this bounty letter" regarding a giant kill. Did that and went back and got another one from the inn, which I also completed. I was level 22 by then, but the Jarl won't offer me land, and the innkeeper hasn't any more bounties for me. What next? I'm on PC so I'm ok to use console if necessary.