No. in each host you will have a disk group. each disk group can have 1 caching device (SSD) and at 1-7 capacity devices (SSD or Disks). You can have 5 disk groups per host. In your case you have a hybrid configuration, which means in order to add "flash" you need to create an extra disk group, which means you also need to add spindles. What type of performance issues are you hitting? And what types of flash are you using and what type of spindles?
Cache capacity not enough，SSD only 240G，I thought that adding a cache disk can be solved.
Why can not become two disk groups after I add one SSD in server. 2*（1SSD + 2HDD）.
Whatever，Can I use one 800G SSD to replace the old 240G SSD，and How？
What method have you used to positively determine that a 240 GB SSD is not enough cache in your situation?
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"Can I use one 800G SSD to replace the old 240G SSD，and How？"
Replacing cache-tier SSD typically requires recreating the disk-group (DG).
As you only have 3 nodes and a single DG per host, doing a rolling evacuation won't be possible (assuming everything/anything is FTT=1), so make sure you have good back-ups and then do do a rolling decommission of DGs, replace SSDs, recreate DGs and then data resync.
Another option (provided your controller supports hot-add/swap) would be to decommission a single capacity-tier drive with full evacuation, use this drive + new SSD to make a new DG, then proceed to do the same for the rest of the capacity-tier drives - this option may be preferable as it would require no temporary reduced-redundancy.
You could also make 2 DGs per host with 2 capacity-drives each but then the cache-tier SSDs in each would be non-heterogenous.
Add disks to a DG:
Do please pay note to the different evacuation/removal options for disks/DGs and ensure the appropriate action is taken (e.g. NEVER use no evacuation option unless you are sure the data or that data-mirror set are not needed.)