Retention Security clause in appointment letter to trainees?

Dear Sir, We have already issued appointment letter to trainees. We didn't mention Retention Security clause.
We would like to retain them minimum for 3 years. Kindly guide.
Many Thanks,
Mili

Dear Mili,
If you wanted to retain your employees (trainees) then you need to improve the conditions of employment, welfare measures, job challenges, management style of the managers etc. If you are HR Manager then please ask question to yourself whether you have done all this. Going further, you need to make their career plan mentor them or groom them so that they handle next level assignment.
Not many company do all this. Rather they retain 1-2 months salary of the employees and call it as security deposit. This security deposit has some lock in period. In your case, you wanted it to be for three years.
However, this is a new condition of employment and well before recruitment, you should have informed about this condition to them. Now if you wish to impose this new condition, you could face backlash. How will you handle that?
By the way, suppose you keep some security deposit with you and what if the employee still wanted to separate forgoing the security deposit? How will you handle this situation? Please note that not all the youth have pressing responsibility at home and their needs are well taken care of by their parents. Therefore, finally it bogs down to improving the organisation's culture and brand image of the company so that you do not resort to these measures which does not have any sanction from the management science!
Thanks,
Dinesh Divekar

Dear Sir,
Many Gratitude for inspiring reply sir.
You are absolutely right sir, we must have to motivate them by rendering excellent work culture.
For getting hike they would like to change very rapidly. After social media came everyone wants less work, more fun,best position,best package in a very limited life span.
Will do according to your guidance sir.
Many Thanks,
Mili

Dear Mili,
As such I have given suggestions in my previous reply. This reply is for your following statement:
"For getting hike they would like to change very rapidly. After social media came everyone wants less work, more fun,best position,best package in a very limited life span."
Yes, what you say is correct. But then please note that it is more than twenty years since remote control became part of the TV sets. Therefore, change started coming at the flick of the button. Then came the mobile phones and now even smart phones with touch screens. If the technology has changed the mindset of the current generation then they should not be held responsible.
The second aspect is about the youth who are entering in the job market. Parents of most of today's youth are beneficiaries of the economic liberalisation. Children were doted and were always given more than what they needed. Therefore, again the current generation is not responsible for that.
What I do not agree with you is about "less work". What they do not expect in their job is monotony. They wanted to be valued. This is because since their childhood even their small achievement was well-recognised. Therefore, if you are able to provide the challenging assignment and recognise their efforts then they will surely remain with your company.
What matters is value addition done by freshers in your company. If a career-conscious youth works with complete and wholehearted fidelity, adds value in his/her two years stay and then quits, then nothing wrong per se.
By the way, let me give you example within our network. A daughter of our acquaintance, completed her MSc and then MBA (Fin) and later appeared for SBI PO Exam and then cleared it also. After training, now she wants to quit the SBI. She says that the work culture in "boring". Parents have their only child. In today's world, will any parent tell her that it would be foolish to do so? Never. In preceding generation it was possible but not now!
Thanks,
Dinesh Divekar

Dear Sir,
Many Thanks Sir for your very substantial reply. They have always required challenging profile. you are absolutely right sir. Even mental peace is there or not. doesn't matter for them.
Gracious,
Mili