iPhoto is still in the Applications folder. It was just replaced in the Dock by Photos. If you were using iPhoto 9.6.1 it will still run under 10.10.3. If it was an earlier version you can obtain it from the App Store by following the instructions in this post by Terence Devlin:
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.
To answer you question about Photoshop, no, PS can't be used from within Photos to edit. Photos requires plugins from the 3rd party editor developers for that to happen. So it's up to Adobe to provide the plugins for Photoshop and Photoshop Elements for Mac before we'll be able to do what we did with iPhoto.