Hi,
yes, this is okay, with this 3 devices settings you are covering all possible devices.
tablet = all tables (except windows 8 IE10 tablets with x86 cpu - there is no way to detect if they are a normal pcs or tablets)
mobile = all mobiles (android mobiles, iphone, ipod, blackberry mobile, windows phones, ..., all devices with 'mobile' in the user agent)
desktop = the default when the device is not a tablet and not a mobile
Best regards,
Klaus