Central Bank of Kenya (CBK) data shows that the three counties have 794 branches or 53 percent of Kenyaâ€™s 1,502 banking outlets.
Nairobi accounted for 39.5 percent of Kenyaâ€™s bank branches, reflecting the capital cityâ€™s economic dominance over the other 46 devolved units created in 2013 to address the wealth imbalance.
Bank CEOs say the spread of banks is informed by cash flow and economic activity in the counties, signalling that the outlets are a bellwether of riches in the devolved units.
The heavy concentration of branches in Nairobi indicates inequality in the countryâ€™s economic development, which has partly been attributed to the previous centralised system of government which guided sharing of resources since independence.
The devolved system of government raised hopes of addressing the economic imbalance, but analysts say there is a need to offer incentives to attract private investors to counties.
The 20 bottom counties have less than 10 bank branches each, with some like Samburu, Tana River and Mandera having three outlets each.
â€œBanks follow economic growth, as result branches are concentrated where the centres of the economy are,â€ NCBA managing director John Gachora said.
â€œAs banks raced for corporate clients in major cities and towns it led to a concentration in areas where they would find those types of clients.â€
The three counties with a heavy concentration of banks are home to 17 percent of Kenyans but they house 53 percent of lendersâ€™ outlets in the country.
Counties like Nakuru, Kakamega, Bungoma, Meru, Kilifi, Machakos and Kisii have fewer banks than Mombasa despite being more populous.
CBK data on the spread of bank branches matches previous countiesâ€™ wealth data from the Kenya National Bureau of Statistics (KNBS).
Nairobi, Mombasa and Kiambu accounted for 30.9 percent of Kenyaâ€™s GDP or total wealth. The bottom 20 accounted for 15.6 percent of the countryâ€™s total wealth.
The Africa Wealth report for 2019 report published this month by Mauritius-based AfrAsia Bank showed that Nairobi accounted for 73 percent of Kenyaâ€™s billionaires.
â€œWe expect Nairobi to break into the top five wealthiest cities in Africa soon, possibly replacing Lagos which has been slipping down on the list,â€ says AfrAsia Bank.
Bank branches have seen little change over the past four years, despite lenders deepening digital and mobile banking.
The number of bank branches decreased from 1,518 in 2017 to 1,502 last year, which translated to a decrease of 16.
â€œThe decrease in physical bank branches was mainly attributed to the adoption of alternative delivery channels such as mobile phone banking, Internet banking and agency banking,â€ the CBK said.
The rise of mobile banking has allowed lenders to reach customers directly, reducing the need for physical locations in a move that has also led to massive job losses among clerical staff. Over the last five years, banks have shed 6,574 clerical jobs from a high of 18,539 in 2014. The move towards digital banking over mobile phones allowed them to employ technology to eliminate mundane tasks, manage costs, and increase efficiency.
Basic services like account opening, over-the-counter transactions and sales, are moving onto digital platforms.
The digital platforms not only provide money transfer but also credit and savings, payments for goods and services as well as e-commerce through linkages with various financial and non-financial institutions.
Mr Gachora, the NCBA boss, said that the gap left by brick and mortar presence was being filled by digital banking, leveraging on the high level of mobile penetration.
â€œFinancial inclusion has been a success in Kenya as most people living in rural areas can access mobile online banking services. Going forward more branches will open and put their focus to sales as opposed to service as most services are now available on digital platforms,â€ he said.
Penetration of formal financial services stood at 82.9 percent from 75.3 percent in 2016, according to the 2019 FinAccess Household Survey jointly conducted by the CBK, the KNBS and FSD-Kenya.
FSD-Kenya noted that the usage of traditional bank accounts had dropped from 32 percent in 2016 to 30 percent in 2019.