Investigations into the attempted murder of Mohammed Khan and the deaths of Arulchelvan Prasana and Rebecca Thompson cost more than £100,000 in total.
The conviction of Michael McInerney - who was imprisoned for his part in the death of Arulchelvan Prasana, who was pushed from a speeding van in Costco car park in 2013 - cost £45,705.29.
The probe into the shooting of Mohammed Khan, who was left paralysed in the attack, cost Hertfordshire Constabulary £43,554.04.
Fesar Khiaq and James Campbell were jailed in 2013 for conspiring to murder the 55-year-old in his Watford home in 2011.
Simon Thompson strangled his 11-year-old daughter Rebecca as she slept in her bed in their Bushey home in 2013. He was sentenced to 20 years in prison, with the case costing the force £17,729.69.
The three cases are in the top ten most expensive investigations carried out by police in Hertfordshire between 2010 and 2015.
The most expensive probe was a murder investigation in Hatfield which resulted in three convictions and cost police chiefs £52,170.12.
The force was responding to a Freedom of Information request by the Watford Observer.
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