These kind of relative-time queries are problematic in any map/reduce database, because the criterion you really want to search for ("created in the last three months") relies on external state, in this case the current date/time.
In the case of Sync Gateway, docs are assigned to channels when they're created or updated, so you can't have a channel for "the last three months' data" because docs would have to leave that channel even when they haven't changed.
What I'd suggest is having a channel for each month. The sync function can parse the date_created and assign a channel name like "2014-06" or "2014-07", etc.
Then the client can configure its pull replication to sync with the channels corresponding to the latest three months. Currently it would use "2014-06", "2014-07", "2014-08".