connect I2C devices with diferent voltages, 3.3v and 5v

hi,

Anyone out there has experience to connect I2C devices with diferent
voltages, 3.3v and 5v respectively? I'm working on a project with
MCU(3.3v) to control three devices, one is 3.3v, other two are 5v, I
have trouble to get it work. my question are:
1. can SCL and SDA be connected directly among those devices?
2. should pull up resistors be connected to 3.3v or 5v?
3. do I have to use voltage translator between 3.3v and 5v?
4. should I use series resistors on SCL and SDA, should they be close
to 5v devices or 3.3v devices?

thanks

.

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