If you want to win all three and don't mind having your bid reacted to by others (e.g. to probe your maximum to bid just above it, or worse just below it), then simply place your true maximum bid on each at a time that is convenient to you and wait. If you don't want others reacting to your bid (e.g. probing your hidden maximum to bid just above or worse just below it) use a "snipe service" to place your one true maximum bid about 6 seconds out; mine allows limited snipes for free or several different pay plans if you are using it more frequently (or want a bit more reliability that comes with a "mirror server" in another location than the main server) and uses an eBay supplied API instead of the glitch user interface. It also captures any error message that eBay returns if the bid is unsuccessful, as opposed to you using the user interface where any such error message is overwritten when the countdown gets to 0 so you generally don't even see it much less be able to read it. Do a websearch of the term "snipe service" as we are not allowed to make unsolicited recommendations of particular ones.
If you are looking to buy only one of the items and they are not ending at the exact same time you can place your true maximum bid on the first one, wait to see if you win and if not repeat with the second one and then if necessary the same with the third. Good snipe services have "bid group" functions where this is automated (but check the separation time required for that service to determine the didn't win status of the earlier auction and then place the bid for the later one, I think mine is 2 minutes between auction ends).
If you only want to buy one and they end at the exact same time, you have to make the choice and commit to only one.
