Is it permissible to purchase a car from the company I work for by paying 20% upfront and the remainder in installments over 6 years, while also utilizing a loan with a 3% interest rate?

It is not permissible to buy a car from a company if the company stipulates a usurious loan, because a sale conditioned on a loan invalidates the contract according to the jurists of the four schools of thought and others. This is due to the benefit for one of the parties that the contract does not necessitate. Al-Kasani mentioned that among the conditions that invalidate a sale is "a condition that the contract does not necessitate, and in which there is a benefit for the seller or the buyer... or on the condition that the buyer lends him a loan." Al-Dusuqi stated in his Sharh al-Kabeer (Grand Commentary), "Like a sale conditioned on a loan." Ibn Hajar al-Haytami explicitly stated in Tuhfat al-Muhtaj (Masterpiece of the Needy) the invalidity of a sale conditioned on a loan.