What are the odds of becoming reinfected a second time with the coronavirus COVID-19, and then a third time? Does resistance lessen with each new reinfection?

Adverse consequences will likely increase. Each new mutant of the virus will have similar infectious potential as the virus from the original exposure for a given individual, but what may work against them is a lowered inner resistance from having their system weakened and their tissues perhaps not recovering fully in rebuilding from the prior onslaught. It is similar to the vulnerability of those with prior lung difficulties in picking up infections in the first place. Each new battering of the lungs from an infection will make the consequences worse, so it is not the likelihood of infection that is the source of worsening, it is the greater vulnerability, in a sense, a loss of lung reserve capacity by not regaining the former robust health and resilience of youthful tissue that will make each reinfection more challenging to live through and overcome.