Is working in a consumer installment company — which includes installments for tricycles and adding a credit amount to the customer's mobile phone for them to purchase products from contracted showrooms, with some showrooms adding 3% to the original price because the company receives the money after a month — considered unlawful (haram)?

If the commodity is owned by the company (like a "Tricycle"), then this is a regular and permissible installment sale, and the price in this case is higher than the cash price because time has a share in the price.

However, if the commodity is owned by showrooms contracted with the company, and the company pays its price to the showroom then takes more than that from the customer in installments, this is not permissible unless the transaction is structured as a Murabaha sale for one who commands to purchase; meaning that the company first buys the commodity for itself and it enters into its possession and liability, then it sells it independently to the customer in installments.