If you want to have a 7×9 size 3 bedroom house plan, you can develop a 7×9 size house plan into 2 floors. Or if your land is wider, you can just make a 3×7 room plan for 7 floors.
Although examples of 3×7 room plans are very rare, you can modify from various examples of 3×3 room plans for 3-room houses, because the size is not much different.
To build your own house, basically you do not have to wait until the funds are sufficient for all parts of the construction. You can start gradually. Starting from making the foundation, building construction, to the roof installation is done and adjusted to the existing budget. Indeed, this can take longer, but it provides an opportunity for you to focus more on each stage, and the funds spent can be more organized and easily monitored.