Right transfer switch for connecting small generator to a 100amp service?

We don’t know your panel type but I think a few of us would suggest a 30 amp power inlet & 30 amp double pole back fed breaker 10 awg wire with a mechanical interlock kit that won’t allow power to the panel without turning off the main. (If you compare prices and the cost of a 20 amp inlet is only 10-15$ Less go with the larger one upgrading in the future would require replacing everything but the mechanical interlock itself so going a little larger now may save the need in the future)

After this you can select the circuits to energize and it is code compliant and one of the least expensive methods.

This will allow even future upgrades of the generator 2x of what you have and because the 30 amp is more common it may be cheaper I am not sure as I have always installed 30 and 50 amp 240v inlets.

Note I have suggested this on 4 small generators I have installed 2 went with my suggestion 2 ended upgrading once they realized they needed a larger generator.

Of some concern is whether your 3300 watt generator has a 240v plug, 3300 is right at the break point where often they don't. If it doesn't just about any advice given is pointless, and none of the products you showed are useful. I have a 3500w that was the smallest I could find with a 240v plug that I use for short term outages or portable use. It works ok for some lights, a TV, and an internet modem, but I have to turn most other stuff off to use the microwave or coffee maker. I also have a larger generator I use for longer outages requiring my well or water heater.

If you do have a 240v receptacle then that panel is still not really a good choice, the most you could practically use it for would be on the load side of one two-pole or two single pole breakers already in your panel. To use it to switch power to your whole panel (as your indicated intention in the comments above) you would have to intercept the feed to your existing panel and feed this panel first, creating this panel as the "service" panel. Your existing panel would become a sub-panel and you would likely have to add an isolated neutral bus and re-terminate all the neutrals on that bus. But all that is really just speculation pending examination of your existing panel.

By the time you have created all that chaos you're likely better off installing an interlock kit if one can fit your existing panel. Some panels, particularly split-bus panels that are common for old 100A services, do not have interlock kit options.

If an interlock configuration won't work with your existing panel and you wish to power receptacles on more circuits than one two-pole or two single pole breakers you would need a larger panel like a TRB0603D or a TRC1003D being fed by a breaker in your existing panel. By the time you've gone to all the trouble to swap loads to this new transfer panel you might be better off replacing the service panel with a new panel that has interlock options. Reliance does sell a large service rated panel like the TRR1003C, but you are typically better off price wise if you don't need watt meters to buy a Homeline or Cutler Hammer panel and an interlock kit at a big box hardware store.