As someone who knows 5 languages and is working on their 6th, I have to say it’s a real bad idea.
There are a lot of similarities in languages, they are all more or less related, learning more than one language at once can and will lead to confusion.
If you are very young, (1yo – 4yo) then you can, you can with ease. But if you are any older than say 10 or 11 years old, you best go one by one.
What you can do, Is study one language, while remaining open to the idea of learning a word here or there in other languages. But if you study all 3 at once you will mess up.
Lets say we are learning English and Spanish at the same time:
In English, we say “it was a big dog”, But in Spanish you say ” It was a(female) dog(female) big” or “It was a(male) dog(male) big” Not only is the order of the words reversed, but “a” and “dog” have variations depending on the sex of the dog. If the dog is male it is “un perro” but if the dog is female it is “una perra.”
Now lets imagine you are also learning German at the same time as English and Spanish:
In Spanish a dog can be a “perro” or a “perra”, if you remove one letter “r” from “perro” and ”perra” you get “pero,” meaning “but” and “pera,” meaning “pear.”
In English a dog can be a “dog” or a “hound,” and in German a dog is “hund” similar to the English “hound”.
Now lets imagine you want to say “It was a big dog” in one of your new languages:
If you now wanted to say “it was a big dog” in your newly learned english, you would have a load of words bashing around inside of your head. The first thing your brain would tell you is a load of nonsense, e.g. “dog, bitch, perro, perra, pero, pera, but, pear, was, era(sp), era(eng), hound, hund, it, el, la, him, her, etc…”
When you go to say “it was a big dog” you could end up saying “it was him hound big” or “her dog was a big bitch” or some other nonsense.