Is zakat obligatory on an amount of 3,600 dinars that was saved monthly from a salary over 3 years and spent entirely on marriage expenses?

If a portion of your salary was withheld with your consent, then its zakat (definition) is due for all the past years. If it was withheld against your will and you could not dispose of it, then it is like a debt owed by a procrastinator. In this case, according to the Malikis, its zakat is only due for one year after you receive it. Zakat is an obligation upon the individual and is not waived by the consumption of the money.