You might not want to burn your bridges right now until you're sure Photos will suit your needs. Photos is much different than iPhoto and is missing many of iPhoto's features - Photos vs iPhoto: Features and Capabilities
iPhoto 9.6.1 is compatible with 10.10.3. If you have it you can continue to use iPhoto. If you have any version of iPhoto 9.x you can obtain 9.6.1 from the App Store by following Terence Devlin's instructions in this post:
Terence Devlin
Apr 14, 2015 11:21 AM
Re: Is Iphoto gone ? i want it back!
in response to Johannes666
Recommended
Go to the App Store and check out the Purchases List. If iPhoto is there then it will be v9.6.1
If it is there, then drag your existing iPhoto app (not the library, just the app) to the trash
Install the App from the App Store.
Sometimes iPhoto is not visible on the Purchases List. it may be hidden. See this article for details on how to unhide it.
http://support.apple.com/kb/HT4928
One question often asked: Will I lose my Photos if I reinstall?
iPhoto the application and the iPhoto Library are two different parts of the iPhoto programme. So, reinstalling the app should not affect the Library. BUT you should always have a back up before doing this kind of work. Always.