Is it possible to get a floating power supply to power multiple MOSFETs
Sharing the one converter is "hard" but there is a low cost and easy and simple alternative (choose any 3).

It is common to use a "bootstrap" supply to high side gates using the switching action to provide the reuaisite "AC pump" - BUT that needs the gates to be switched more often than the decay time of the voltage which is produced in one switching cycle.  So ...

Have a look at the simple but useful concept used in this SE Q&amp;A .
An AC square wave (can have asymmetric mark/space) is capacitor coupled to the floating node. A +ve half cycle conducting diode provides charge voltage and a negative half cycle diode to local "FLOATING GROUND'/ source" maintains the capacitor voltage at 0V-floating on average. 

As seen here it is used to directly drive the gate BUT the same method could be used to provide a floating voltage which is then used by an appropriately coupled floating driver.  Do read the answers and comments as they address several relevant issues.

Here are two conceptual circuit diagrams.
Design is required.

The upper circuit provides both drive and drive-power by applying a square wave to "AC signal drive".   

The lower circuit uses the diode pump to provide a floating DC voltage and a separate signal line - derived by whatever means suits - perhaps an optocoupler, or ... .







simulate this circuit &ndash; Schematic created using CircuitLab

This circuit can be used to provide floating voltage for other applications.

In this application and (probably) yours the gate is driven relative to a steadyish source voltage which is DC referenced to the local ground. If the circuit is fully floating then a capacitor can connect floating ground to local ground or an 180 degree out of phase drive input can be provided by two capacitors. Some low cost panel meter circuits use this arrangement to 'float' the meter supply. Note that when you have multiple driven FETs "standing on top of each other" the switching of one stage may affect others - this needs to be accounted for in  the design detail. 

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Also see SE questions gate driver and   [floating gate driver](https://electronics.stackexchange.com/search?q=floating + gate+driver)

This graph and circuit are copied from Selvek's excellent answer to the above reference question.
It can be seen that the gate voltage acquires it's full value within a few cycles of the AC drive waveform. If you wish to use simple high/low drive signals from the microcontroller you could AND these with an appropriate AC waveform - or the uC can provide the AC signal. Do read the cited Q&amp;A as voltage drive levels, gate turn on voltages and more are discussed.