I am in awe of anyone who can speak multiple languages, and Croatian is one of my favorites - My partner is 1/2 Croatian and his father can still speak a little bit. It's a beautiful language, so....kudos!
I don't know if you are too much of a beginner, technically 3 weeks says yes, but if you learn as quickly as it seems, it is quite possible you could tackle the Academy. Did you try his free sample of the course? It is offered on the website at the very bottom, I think.
So, normally the price is steep, $97 a month (waaaaay out of my league) - but it seems that when room in the course opens up, he has a quick one or two day sale for people on the list - $35 a month.
So, my best suggestion is download the free offer to get an idea of it, and if you like it, hop on the waiting list.
If you think it is too advanced or even too pricy at the sale price, there is a great (Well, I really like him) guy, Gringo Jordan, who has tons of free videos, and SO many on grammar. The first link is to his YouTube page, just scroll down a bit and you will find them:
And he has a very inexpensive "member" page that has a verb conjugation boot camp that absolutely kicks ass. I have a love/hate relationship with it, haha.
Some people like me love his style, others hate it.
But he is worth checking out for the tons of free grammar content that is really pared down to the basic patterns, and it takes the "Oh dear GOD" grammar dread away quite a bit - again, this is just my opinion.
I need uncluttered explanations because I was horrible with English grammar in school, and I totally walked away from it. So when I started Spanish, I was" WTF is the subjunctive past in English, for chissake???"
¡Espero estas son útiles, y tienes un gran fin de semana!