Shall I keep Avast Internet Security, McShield2 and Mawarebytes all working simultaneously? Or will they interfere with each other / slow down my laptop?
No, do not worry. They shall not interfere with with each other as Malwarebytes adn MCShield, they are not AntiVirus program.

While AntiVirus program is the big boss, MBAM and MCS are AntiMalware tool, they shall assist AV as additional protections.