From what I've heard most people using/liking these agentic IDEs are using Claude models to power them, they seem to be the best at writing code. By buying Windsurf (and trying to buy Cursor) OpenAI can figure out why Claude is better at this task, fix GPT, and then make GPT the default for the coding use case.
Not sure it's worth $3B, but that's also not a lot to them when they can raise unlimited money at any time,