Why just not rent a 3090 from vast ai? This is literally 0.2$ per hour.
Vast is a grey-market with very few security measures in place. A host can snoop on your workloads very easily for example because they have full access to the docker host.
You're also limited in that you can only deploy a single container so if you just wanted some spot nodes to scale out your existing Dagster cluster etc.. You're not going to be able to cleanly plug Vast into your existing infrastructure deployment process.
But if you're just playing around or don't have major security/data restrictions and only want a jhub notebook or nvidia-glx-desktop then yeah it's a very hard deal to beat.
So unless you have stable tasks that need to run continuously, or have enough users to keeps your GPU clusters busy, your GPUs usage would be quite bursty: some period of high activity then a lot of time idling.
When you're just messing around, this is great. Home brew gpu labs are fun and cool. As soon as you bring economics into it and start valuing time, it's a non-starter.
And your electricity bill alone would be $100. So you'd be spending $1100 over those 3 months purchasing the card, -vs- spending $432 in the cloud.