is there any way of installing or getting the update for IOS 9 beta 3 because i have the IOS 9 public beta 1
is there a way of installing the IOS 9 beta 3 i already have the IOS 9 public beta 1
No, the Public Beta and the Developer Betas are not interchangeable.
To get the Dev Beta 3 you'd have to Buy an Apple Developer Membership, restore your phone to 8.4, and follow the steps for installing the Dev Beta 1, from which you could upgrade OTA to Dev Beta 3.
In addition to what Max108 said, your question is flawed because public beta 1 and dev beta 3 are nearly identical anyway. Now mark this question as answered please.