How can I wirelessly connect my 2017 imac to my older imacs?

You have a few of options.

1. Have the 2017 iMac provide a wireless network to the old Mac by sharing its own internet connection.

macOS Sierra: Share the Internet connection on your Mac

2. Wire the old Mac directly to the router if at all possible.

3. Get a wireless router (does not need to be an Airport Express or Extreme) to connect to your modem/router and provide a wireless network to the old iMac.

If you aren't using the Wireless network provided by the modem/router, I would turn it off to avoid conflicts and interference if you choose to use a different wireless solution.

You can share the printer connected to the 2017 iMac to the older iMac via the wireless network in cases 1 or 3, or via the wired network for case 2.

macOS Sierra: Share your printer

You need a Wi-Fi network with all devices connected on it and that's about it. If you want to share files the most simple method of doing this is setting up iCloud across all devices. If you don't have experience with iCloud then please begin with Set up iCloud on your iPhone, iPad, or iPod touch - Apple Support If you want to print simply attach the printer to the Wi-Fi network and ensure the version of Mac OS installed on each Mac supports the printer.

Is your internet provider's router offering a wifi network to connect to?

How is the 2017 iMac connected to the router?

Is your wireless printer connected to your router? How? Wirelessly?

If your 2017 iMac is connected wirelessly, and your printer is connected wirelessly, can you not connect the 2011 iMac to the router in the same way?

For number 2, what are you looking to do with them connected exactly? Have the 2011 access the 2017's iTunes library?

Have it access its files?

You do not specifically need an Airport device. If your router is already putting out a wifi network, you can use that.

If there some other reason you can't d that, you are going to have to tell us what that is.