LOTS of things could cause this issue, many of which won't be identifiable on the livestream PC, and especially not from OBS Studio logs
also - no logs/attachments in your post
7000kpbs is what we use for our HoW livestream of a single 1080p30 stream to Facebook. 3 (or 4) streams in that same bandwidth is going to be low quality.
And is your 50mbps upload on fiber? if cellular, then jitter, latency, and natural variance in throughput is to be expected
what other traffic is using that link? if you allow Guest WiFi, if you are like most locations, you have no idea what Guest traffic looks like (which could cause contention)? Does your network have QoS settings to Prioritize Livestream traffic and/or make sure other traffic doesn't contend?
Are you using the exact same stream encoding settings for all 3 targets, or are they unique (meaning 3or 4X rendering workload for the PC)?) OS and driver updates can change performance. If using recommended (unique) encoding settings for each CDN stream, I'd be more inclined to use restream.io or similar (stream to single target, let service replicate stream)
side note: Personally, I prevented multi-streaming, as it dilutes sense of community. And in our case, community was on 1 platform primarily anyway, though in last 5 years that is changing, so possibly we will change? We do Record locally (at much higher bitrate) and now upload to other services later (not livestream). Unless the congregation is huge, multi-platform livestreaming makes no sense to me.
