How to switch straight to a keyboard in mac
One easy thing you can do is to setup a shortcut that cycles forward and backwards through all input sources you have on your Mac. 

So assuming you have them setup in this order: Japanese, English, Chinese and Japanese is currently active, pressing the keyboard shortcut will move to English and if you press it again to Chinese and then back to Japanese after a third press. With the second shortcut you can cycle backwards at any time.

Alternatively you can set an unused F-key in the top row of your keyboard (on mine F5 and F6 is free and on a full-size keyboard you have F13-F19 unused too) to be your shortcut, so pressing those will cycle through your options. 

As far as I know there is no solution straight from OS X that will allow you to pick a specific language just by pressing an F-key. What might be an option is to setup an Apple Script inside System Preference that is being triggered by an F-key which sets your input source to a specific language. I might read into that and post it here, if the previous option doesn't meet your needs.

How to set it up


Go to System Preferences (see Apple Logo at the top if it's not in the Dock)
Go to Keyboard
Select the tab "Shortcuts"
Pick "Input Sources" from the list on the left
Activate the shortcuts you would like to use and then click on the right to define the shortcut by pressing the desired combination or F-key.
Close System Preferences and try it out


I hope this was helpful to you.