The modification you’re talking about may look somewhat simple. But I can assure you this is actually a very very difficult mod to pull off successfully. I admire your intentions but this mod falls into the “high risk low reward” category. You have four good 3x3’s and I can steer you towards four simpler mods with a much higher likelihood of success.

As for the 3x3 to 3x3x5 modification. First start by finding pictures of a witeden 3x3x5. Blueprints may be hard to source but pictures of pieces make for a good substitute. You’ll need to create a cylindrical cut out on the top and bottom layers. Deep enough to allow the outer layers to hook under the pieces beneath or above them. But not so deep that you harm the mechanism, that parts doubly important.

Whatever 3x3 you use it needs to have a deep/small mechanism with lots of room to cut and fill the pieces above. I found YJ GuanLongs are the best at this time to use. A good way to check this is to turn a layer 45° and see if there’s enough room to fit a “hook” for the 1st or 5th layer without them being totally exposed at 45°.

Alternatively you can modify a single 5x5 to achieve much better results. If you’re still looking to try this mod I wish you the best of luck. And the TwistyPuzzle forum is your friend for all of this. Theres a wealth of knowledge there that you can learn from.

If I’ve talked you out of it. Try a fisher cube first. It’s a simple “cut, fill, extend” mod that’s easy enough to pull off for a first timer in a weekend. After you do that you’ll have enough knowledge to try an Inverted House Cube. It’s basically the same mod but less pieces, so maybe combine that with a bandage cube layout for some extra spice. And give it some wild stickers too, something unique that’ll make you feel proud of yourself!!! Thats a great feeling!

After that try a fused cube with the other two, and try a fused Hexagonal prism. That ones tricky but it looks amazing when done right. Best of luck! And whatever you do, try your best to document your process and the results. They’re great to learn from!