What happens to my 3-month visa if I travel to Laos after 1 month?

You may be confusing the validity of your visa (90 days from issue) with the number of days you’d be stamped in to Thailand - which if it’s a tourist visa - would be 60 days.

Without knowing the visa you hold it’s not really possible to advise correctly, but whether a Non O or a tourist visa it is used up on your initial entry. It’s finished. Gone.

What matters is the days you’re stamped in for. In either situation the number of days you have left would be lost if you left the country without a reentry permit - which would keep that date stamp “alive”.

If it’s a tourist visa that you have then it may not be worth you getting a reentry permit as it will cost you 1,000 baht.

Assuming from your profile you have an Israeli passport then you are eligible for a 30 day visa exempt entry for free.

Three month visa? I think you may have the 60 day tourist visa, (valid for entry for 3 months), which you then pay ฿1900 to extend that 30 days, giving you a total of 90 days.

If that is the case...

If you are leaving after just 30 days, you could pay for a entry permit, I think circa ฿1000 and you'll have the same stamp date, stamped in when you come back. Note having already used 30 of your 60 days and depending how long you stay out of Thailand (lets say a week) then you'll only get stamped in for 22 days on your return. Where as if you just left and returned WITHOUT paying for a reentry permit, you'd get another 30 free

If you do have a non-oa or non-o 90 visa, the none of the above applies.

You need to get a re-entry permit to keep your 90 day stamp

What visa do you have? Regardless your entry stamp will be cancelled when you leave unless you purchase a re-entry permit before you leave.