"I couldn't but laugh" - correct or not?

Is the usage of "I couldn't but laugh" in place of "I couldn't not laugh" correct English? Or should it always be "I couldn't help but laugh"?

Solution 1:

I couldn’t but laugh means All I could do was laugh, implying that there was no practical action I could have undertaken (or, at least, that’s how older members of my family use it; it’s outmoded for me and I don’t use it).

By contrast, I couldn’t not laugh means I couldn’t help laughing, I had to laugh.

So, the two sentences are not equivalent and shouldn’t be interchangeable.

The third sentence, I couldn’t help but laugh means the same as I couldn’t not laugh.