Just look at the bit rate of what you are streaming and multiply it by 3 then add a little extra for overhead.

I have 35mbps upload from the ISP, and limit each stream to 8mbps. This covers direct streaming all my 1080p content and a 4K transcode as needed.

My family is very satisfied with 6 mbit/s per stream. Some HEVC, most H264. They see it as high quality. 3 Streams would be 18 to 20 Mbit/s

Are you transcoding?

4mbit per client for 1080 is generally a workable minimum for the average casual watcher if you have H265 compatible clients (and a decent encoder, like a modern intel CPU for example), 6 - 8mbit per client if its H264 only.

Remember that the bitrate to quality curve for live transcoding isn’t as good as a slow, non-real-time encode done the brute force way on a CPU. so if you have a few videos that look great at 4mbit, dont assume your own transcodes will look quite that nice, you’re using a GPU to get it done as quickly as possible, with acceptable quality, not as slowly and carefully as possible for the best compression.

How expensive is internet? If its cheap go overkill and don’t worry about it.