'Is it ok to run a plm fixed effect model and add a factor dummy variable (tree way fixed effects)?

Is it ok to run a "plm" fixed effect model and add a factor dummy variable in R as below?

The three factors "Time", "Firm” and "Country" are all separate indices which I want to fix all together.

Instead of making two indices in total by combining "Firm” and "Country", I find the below specification works much better for my case.

Is this an acceptable format?

plm(y ~ lag(x1, 1) + x2 + x3 + x4 + x5 + factor(Country), data=DATA,
    index=c("Firm","Time"), model="within")

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Solution	Source