It will work for a while until the arowana (should we assume silver?) will grow big enough to place it on its potential menu. Might never happen given their lifestyles and the raphs defensive mechanisms but it too depends on the tank set up - size, furniture, lighting, etc.
Also, the better question you ask, the better answer you get in general. I mean we have to guess too whether you are talking about a spotted raph, which grows smaller, or a striped raph that grows bigger.
But if the question is about right now, this moment when they are both 3", then you told us all we need to tell you yes, right now they are quite compatible.
I feel like this is a bit of a charade. I love charades. Have I covered all the main possibilities yet?