If these are standard Semester 1 subjects, then the census date (i.e. date after which withdrawals still mean paying the cost of the subject) will be March 31st, so you should be fine. When you withdraw on your study plan it'll take you to a confirmation screen that has the census dates there anyway.
You should absolutely be able to enrol in Italian unless it's a quota subject or anything like that. I'm prone to enroling --> changing my mind --> withdrawing --> re-enroling --> changing my mind again --> withdrawing --> enroling in a different subject --> reevaluating all of my life choices --> withdrawing --> enroling in a bunch of 'real' subjects like 'Corporate Finance' --> promptly realising my mistake --> withdrawing --> enroling in a bunch of fun sounding subjects like 'Poetics of the Body' --> realising I should probably find some sort of middle ground --> withdrawing --> finally going back to the subject that I'd chose in the first place... all within the span of maybe a week

Worst case scenario, Italian is full or it won't let you enrol, so you can just switch back to Econ and then go to your Student Centre once classes start and get them to manually put you into Italian, if possible. Or just leave your study plan as it is with three subjects, as this still makes you a full-time student, meaning you're still eligible for public transport perks and whatnot. Then a Student Centre person can sort it out for you. If they're just regular subjects though, you should be fine doing it yourself
