Not a good idea, your battery charger will get very confused by the current draw of the 3.3V thing.
Assuming we are talking low current devices then the simple solution is two schottky diodes from battery+ and the 5V rail to the input of the 3v3 regulator. That way when 5V is present the 3v3 regulator will use the 5V source, when there is no 5V it will use the battery.
The down side is that you lose a 0.2-0.3V which for 3.3V doesn't give you much margin when running from a 3.7V nominal battery. If you can drop the voltage on your 3.3V thing to 3.0V then you get a lot more headroom.
You can avoid this voltage drop by using a FET connected as an ideal diode, replace the diode from the battery to the regulator with a p channel MOSFET, connect the gate of the MOSFET to the 5V rail. When the 5V is high the FET is off and the battery doesn't power anything. When the 5V is low the FET is on and the battery is connected to the regulator with minimal losses.
Warning - I am assuming you are using a LiPo battery with built in protection circuit rather than unprotected cells. Unprotected LiPo cells are very dangerous and prone to unscheduled highly energetic self disassembly if you do anything that they don't like. You should never connect them to anything you have designed unless you know exactly what you are doing.
Note - This is all assuming we are talking about a simple solution for a home project where simplicity is more important than maximizing functionality. There will be issues with low temperatures and without a proper low voltage shut off there will be long term damage to the battery that will reduce it's capacity over time. For a full commercial product this entire approach is inadequate and you need to use a more advanced method of power management.