Is there a 3-month entry rule for DTV visas in Thailand?

The rule you have identified is more than likely the Embassy's way of avoiding an avalanche of premature applications.. meaning they only want you to apply if you are intending to travel within 3 months.. in essence prioritising their resource to the most urgent cases.. however the visa isn't granted on the condition that you do travel on the date you stated.. after all, situations change and people cannot always fulfil their intended schedule.. this rule simply deters people from applying too early and creating a back log of applications.

There's a 3 month rule, yes, but it's NOT for the DTV, rather for other visas, including the single entry and multi entry tourists visas (for people from countries which aren't elegible for the visa exemption entry).

It's not a rule, as you've proven. You get 5 years to use the visa.

There's no 3 month rule. You have 5 years to use your visa.

Visa should be used in the period, mentioned on the visa. Nothing more.v

It's strange, a non-existent rule really doesn't exist

I don’t believe there was any rule stating this. Some embassies may have stated that was the case but it’s a 5 year visa. You have to use it within that time.