Competent means having the necessary ability, knowledge, or skill to do something successfully. Capable means having the ability, fitness, or quality necessary to do or achieve a specified thing. Qualified means officially recognised as being trained to perform a particular job; certified. Personally, I feel that competent and capable could be interchangeable when speaking about a person who has the skills necessary to perform a job well. When looking at qualified, the person would have to have gone through some sort of training, or have a certificate or degree to prove he/she has the skills necessary. The best adjective to fit your sentence? I would say qualified because you have mentioned certificates. With so many certificates, I think he is a strong applicant and is qualified for the job. You could, of course, use any of the above words: With so many certificates, I think he is a strong applicant and will be competent in the position. With so many certificates, I think he is a strong applicant and is capable of doing the job.