When it comes to comparing between these colleges, you should know that NIT Delhi is not in the same leagues as NSIT and DTU. NIT Delhi is a relatively newer institute and did not have a campus of its own for quite some time. 

The following points are based on my interaction with various friends studying at these institutes:

• DTU has a really good placement record as far as the core branches are concerned. The mechanical branch here is considered to be the best in Asia. 
• If you are interested in computer-related placements, NSIT has better placements as it has low competition and better proximity to Gurgaon.
• On the other hand, DTU will have a lot of high competition because of its large intake. This is almost double that of NSIT so you will have to beat more students to get your dream placement.
• This is not the case in NSIT. You will see fewer branches and hence, you will face less competition.
• At NSIT, you will see that a Delhi University degree is provided to you. This is will increase your chances of bagging a job off-campus if you plan to do so. 

If you compare NSIT and DTU, the placements depend on your branch as well as the GPA you score at the end of the 6th semester. The following table shows the overall placement statistics at these institutions:

Being a fresher, you will always be lured towards the placement packages. However, in reality, no one really knows the actual packages offered at the institutes.