Sorry, I know you're new, and I'm trying to be nice here, but those are all terrible options sorry.
Firstly don't create a RAID 0/striped array for any data you care about - WHEN you lose one SSD you'll lose all your data, R0 has it's purpose, usually for holding data you don't mind losing like logs, but nobody uses it for professional work, nobody at all. Use RAID 1, if the space isn't enough buy larger.
Secondly are you considering using the third HDD as essentially a backup for the first and second? If so then that's a bad idea too - you shouldn't keep a backup on the same server, if someone hacked into your system they could delete the main data AND the backup, and your business is dead - backup to something else ok.
Also you just say 'merge' for your first and second HDD, again don't use R0 for this please, for the same reason as your SSDs, use R1/mirroring (or R10, R6 or R60), if you're wondering what to do with the third disk perhaps configure it as a hot-spare, that'll help when one of the disks dies.
You mention an optimal solution - Have your boot drive sized for what you want using a pair of SSDs of the right size, configured for R1, then also use R1 for your HDDs and backup somewhere else (btw rsync is great for this, just to another server).
One final thing, you say you have 250GB SSDs, I'm not sure HPE sell 250GB SSDs, tell me you're not using non-HPE disks please?!?