Is it forbidden to receive a salary from two companies for the same three days, knowing that I was absent from the first company during these days, and I considered this a compensation for entitlements (overtime and medical expenses) that I did not receive from the first company?

You must adhere to your work until the end of the contract. The salary taken for days of absence is not permissible unless the employer agrees. As for taking a portion of the salary equivalent to the right withheld from you by the company, this falls under the issue of "al-Dhafar" (reclaiming a right), concerning which jurists have differed.