1. I think they ever don't brush their teeth - this sentence is grammatically wrong it would be best to change it to "I don't think they ever brush their teeth" - the "ever" here is being used as an intensifier do say that they don't just NOT brush their teeth, but NEVER brush their teeth (making it more intense) 2. They don't seem to brush their teeth - This is using the phrase "seem to" which is a less direct way of expressing an opinion. Mostly we would say "seem to" so that we're not making an "assertion" (an opinion stated as a fact) because that it the wrong thing to do most of the time. 3. It doesn't seem like they ever brush their teeth - This sentence is very similar to 1 and 2. The difference between any of these sentences is that 2 is using a plural subject and 1/3 are using singular subjects "I/It" but functionally the sentences are all very similar in their use of Modal verbs (seem to) and opinion indicators "think"

Apart from the grammatical.issues mentioned, there isn't really a difference in meaning. No 1 is more personal, and no.s 2 and 3 are expressed in impersonal form, but everyone still understands that it is the individual's personal opinion that is being expressed.

The first sentence doesn't actually make sense. In order to make it appropriate we'd have to switch some words around and make it into "I think they don't ever brush their teeth." If we take this into account then yes, all three sentences have the same meaning. Every sentence explains that someone must not be brushing their teeth, no matter how it's worded.