Generally 1 Worker Process should be good enough as it is mainly used for general server work like routing requests, SSL decryption/encryption, caching, rewriting, serving static contents, etc.
If you only have 3 domains, the Site Owner Plus plan is good enough for you as it supports up to 5 domains.
What is more important will be the LiteMage Cache license as it depends on the publicly cached objects of each Magento store. Publicly cache objects are stored based on both URL and cache vary. If your site uses a layered navigation menu for example, each combination of selections from the layered menu will be its own publicly cached object. If your site uses any cache varies (multiple currencies, a separate mobile view, or multiple user groups), the number of publicly cached objects in your store will increase in relation to that. Having two currencies, for example, would double the number of publicly cached objects in your store, having three would triple it.
If you have have less than 1500 publicly cache objects for each of your store, the LiteMage Starter plan is fine but otherwise you may need to go for LiteMage Standard or LiteMage Unlimited.
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