You can use modified Van't Hoff equation to calculate the osmotic pressure for binary or tertiary mixture. It is just an expansion of Roult's Law i.e. total pressure is the sum of partial pressures:

Osmotic Pressure (𝛱)=𝑅𝑇Σ𝜑𝑠𝐶𝑠,

Since Van't Hoff equation is applied for ideal case where osmotic coefficient (𝜑𝑠) is ~ 1 but for your case you might have to find osmotic coefficient and that can be calculated based on activity coefficient.

Not sure if this helps.

I think this book Chap might help further:

Chapter Osmosis and Osmotic Pressure

Thanks Dr. lavaraj it was really helpful.

That's a good question