It seems you don’t know anything of building speakers yet. No problem, but in your case, you'd be much better off with ready made speakers. Isn't here a second hand market in Brasil? That Hitachi amp isn’t all that bad.. So start with speakers, then save some money to buy an amp later on, possibly also second hand.

If you really want to use those car speakers, you need to design a box around them, with exact dimensions made for your speaker combo. You also need a passive crossover (and dial it in). Building a speaker is much more than just putting them in a wooden box with mineral wool. There’s software to help you in the process, but it's pretty geeky. See link below.

Your last question: you’d need a stereo amp, so with 2 outputs. With your 3 (x2) loudspeakers, you’d have to build a 3 way loudspeaker box (which is even more difficult than building a 2-way loudspeaker, made of 1 woofer & 1 tweeter). A 3-way loudspeaker box needs 2 crossover filters. The role of these filters is to supply each speaker with it’s own part of the audio spectrum, they need to be dialed to exact values, matching your speakers.

If you buy a ready made loudspeaker box, all that work is already done.

https://listoffreeware.com/free-speaker-design-software-windows/