Can you get std if you have sex with different people but use a condom ?

Lower risk, but yes: Properly used condoms that don't break are very effective against STDs transmitted in fluids (gonorrhea, chlamydia, HIV, hepatitis B) but less so against skin-to-skin transmission (herpes, HPV, syphilis). And mistakes in use are common. Congratulations for always using condoms -- keep it up! But you'll still get HPV for sure and maybe others. You still need HPV vaccine and regular STD check-ups.