Given that the Ninja products typically have electronic controls, I believe you would be safer with what is called a "pure sine wave" converter. There are lots of cheap 'converters' on the market that advertise a higher power rating than they can actually deliver for a sustained time, but they don't explain that. A rule of thumb is that for a 3kW continuous load, you need a converter rated at 6kW intermittent.
If you are considering one of the budget converters, you would need one rated at least 5kW/5kVA. Example:
TBH I wouldn't recommend these gadgets, and reviews suggest they can be problematic.
It would be far safer to use a transformer (or "autotransformer") to step up the voltage from 110v to 230v.
I see an example on the market (eg from Screwfix) that is rated 3kVA 'intermittent' (kVA equivalent to kW for this resistive load), which they say is 1.5kVA continuous, and that is c£130. That's not powerful enough for your Ninja!
A proper step-up transformer rated for your load is likely to cost about as much as the Ninja, unless you can find one secondhand.
The cheapest I can see after a quick look is this:
I don't know enough about it to recommend it, but it gives you an idea of what is required.
You might be better off buying another air fryer at your destination!
Some countries nominally on 110V do have split phase 110 - 0 -110V supplies (ie 220V) available on certain outlets, eg in the USA. Presumably that's not the case where you are going?