I see on Amazon 3 nest products,
1. Google nest wifi router 3 pack 4x4 ac2200 mesh wifi routers-$345.10
2. Google nest wifi ac 2200 router and add on access point mesh wifi system-$239.90
3. google nest wifi mesh router ac2200+ abd 2 nest dual band wifi accesss points with google assistant-$279
can someone explain the difference
I'll do my best to explain, but it is a little complex, so if you have questions about the following, don't hesitate to ask. First, there are two different types of hardware here: "Nest WiFi Router" and "Nest WiFi Point". The Router hardware has a more powerful WiFi radio than the Point hardware, and it has two Ethernet ports (more below about why that's important). But, it doesn't have the smart speaker features that the Point hardware has.
Second, there are two different "roles" that hardware can play in a complete system. One "primary" unit connects to your internet service through an Ethernet cable. It acts as a firewall that creates a new "inner" network. To expand the usable coverage area, one or more "secondary" units can be added to the system after that. They don't need to be connected to anything via Ethernet, but they do need to be close enough to the primary to get a strong connection to it for themselves (typically one or two rooms away in any direction). From there, they can provide WiFi coverage to more distant client devices. For maximum coverage, the primary should be as close to the center of the home as possible.
Ok, so, we have Router and Point hardware, and we have primary and secondary roles.
Here's where it gets a little more complex. Router hardware can be configured in either primary or secondary roles, but Point hardware can only be configured as a secondary role (since it doesn't have Ethernet ports).
So, with all of that as context, the most expensive package looks like three Router units in a bundle. You'd deploy one of them as a primary (connected to your internet service via Ethernet) and the other two as secondaries. You wouldn't have smart speaker features in any of them, but since they all have AC2200 4x4 WiFi radios, the system performance should be pretty good if deployed as recommended.
One of the less expensive packages appears to be a bundle of one Router unit and one Point unit. So, you'd deploy that Router unit as the primary and add the Point unit as a secondary. Since the Point unit only has an AC1200 2x2 WiFi radio, performance for devices connected through that secondary would be more limited, but depending on how big your home is, it might still be just fine.
The other package appears to be a bundle of one Router unit and two Point units. This would let you expand the coverage a bit more than the two-pack option – similar to the three-Router bundle, but with somewhat less peak performance due to the lower performance WiFi radios in the Point units.
I hope this helps you make sense of the options and why the prices are different between them (e.g., three Router units is more expensive than one Router plus two Point units in a bundle).