Can I re-enter Thailand after leaving for 40 days if I've already entered by land twice this year?

You left 40 days ago and you're coming back in to thailand by LAND? Are you in a neighboring country or what?

There is no longer a 2 free entry by land border in a calendar year rule

BUT

that doesn't mean that they don't look at entry history when you come into the country (especially by air).

All you can do is have proof you're flying out of the country in 60 days (because that's what they'll ask when you go to stamp in if they are going to hassle you for entry/stay history)