Edit email html to look good across several email clients and devices

I need help with EMAIL html coding, I already have the code but I need someone to clean it/edit to make it look good across several email clients/devices, the most important ones Outlook 2021, Iphone, Gmail all devices only newer versions. Doesn't have to be coded for responsiveness more than it already is.
It's a 3 column layout.
You will need to have an email html preview feature like Litmus or similar.