I don't have any experience with FiveM or really with most game servers other than MineCraft, but I have a few suggestions for you in order to be able to upgrade the way you are saying:
Motherboard or base system - buy something that is suitable for the type of CPU and amount of RAM that stage 3 will require. You can't really upgrade this.
CPU - either buy a suitable CPU for stage 3 from the start, or buy the absolute cheapest CPU for your platform to start with and then upgrade it. I recommend buying the CPU you will need from the start, because you aren't likely to be able to sell that cheap CPU for anything close to what you bought it for.
Memory - to start with, you either want the cheapest memory you can find (say 1x or 2x 4GB sticks) or you want 1x or 2x of the best memory your platform supports. For example, let's say stage 3 will require 64GB of RAM, and your platform supports 4x 16GB RAM sticks. In this case you would need to fill every slot eventually, and so you do NOT want to start out with any 8GB RAM sticks because they aren't going to be useful in the long run. Instead you would want to start with 1 or 2 16GB sticks and then you can add the rest later without have to replace anything, or you want to start with cheap RAM that you aren't going to feel sad about having to remove.
Storage - install the OS on a cheap but decent SSD. You don't need blazing fast performance. For stage 1 you can install the game server on the same SSD, for stage 2 you should buy a separate storage device(s) to put the game and mods onto, that has enough space for stage 3.