How long will iPhone SE 3rd generation be supported?

Apple normally supports a product for 7 years after it is discontinued, but predicting the future is difficult.

The 3rd Gen SE is still a current product.

Use this as a guideline: Obtaining service for your Apple product after an expired warranty - Apple Support

It also depends upon what exactly you mean by "support". While some model phones can no longer get the newest iOS (17), iOS 16 -and even 15- does get some security updates. My SE(1) got one a few weeks ago.