1. We see India as a strategic partner in the late 21st century.

2. We see India as a strategic partner in the mid 21st century.

3. We see India as a strategic partner in the early 21st century.

(Are all grammatical? Can we use 'middle' instead of 'mid'?)

#2 = mid-21st century. "middle" is a noun = middle of the 21st century.

Sra

1. The sentence "We see India as a strategic partner in the late 21st century" is grammatically correct. It means that the person or group speaking views India as an important ally or collaborator in the latter part of the 21st century.

2. The sentence "We see India as a strategic partner in the mid 21st century" is also grammatically correct. It expresses the belief that India will play a significant role as a strategic partner in the middle part of the 21st century, roughly around the year 2050.

3. The sentence "We see India as a strategic partner in the early 21st century" is grammatically correct as well. It suggests that India is seen as a valuable strategic partner at the beginning stage of the 21st century, which generally refers to the period between 2000 and 2010.

Regarding your second question, it is more common to use "mid" rather than "middle" when referring to a specific point in time. Both words can convey a similar meaning, but "mid" is often preferred in this context. So, the usage of "mid" in the second sentence is more appropriate.