I have a laptop charger which has 19.5 V 3.33 A output. I want to use it for raspberry, which needs 5V and 2.5A and for monitor which needs 12V 3A. Is that charger strong enough to run the whole system? Is it possible to convert 19.5V 3.33A with a buck step down to 12V 5.5A which will be connected to monitor and then use an another step down to 5V 2.5A to feed the raspberry, or I have wrong idea about everything? Sorry I do not know much about electrical engineering. Thanks!
Yes, it is possible
power of laptop charger is = 19.5 * 3.33 = 65 Watts
your loads power:
raspberry pi = 5*2.5 = 12.5 watts
Display = 12 * 3 = 36 watts
total power = 48.5 watts
Total power < power of charger,
you can use it. it will work
The answer to your header question is NO!
Input 19.5x3.33 = 64.935 Watts.
Output 12x5.5 = 66 Watts.
So your new output power is higher then your input power which is against the law of conservation of energy.
But according to your question details your equipment requires only 48.5 Watts. That is do-able but only if your buck step down converter (or converters) has/have at least 75% efficiency.
But as Chris Stratton already said: it is better to use supplies which are dedicated for the circuits. You will be spending time and effort on something special, whilst there are cheap, ready made solutions. The idiom "penny wise, pound foolish" comes to mind.