Assuming that you have 3 bathrooms in your home, that doesn’t mean that a filter that states that it can cater to “1-3 bathrooms” would meet your requirements. That is, because, it is entirely plausible that the size of your bathroom’s faucet might be different than the one which the filter took into account when measuring its flow rate.
Therefore, the best way to size a water filter is to measure the flow rate (in gallon per minute or GPM) of all the faucets in your home. Afterward, compare it with the GPM which the filter claims to supply. Only buy those devices whose rate is at least 0.5GPM more than your requirement.