It will work ( raid 5 needs at least 3 drives), but you shouldn't be mixing hdds and ssds. But in the case of raid 5, all but one of the drives will be working, whereas the last one will be used for redundancy when one of the active drives fail. So in your case, your 2 ssds will be active and your hdd will be inactive and be used for redundancy when 1 ssd fails.