I presume the shareholder has given request for consolidating his share certificates.
As you know, on each share certificate, the distinctive number will be written. Basically, Share Certificate number and Distinctive Numbers are different. By changing the share certificate number, the distinctive number wont change.
The last certificate number of the company would be 20,000.
In this case, for consolidating the aforesaid 3 certificates, first the company should cancel the old share certificates (No. 12, 15 and 18) by writing a remark "canceled due to consolidation on ____" and then issue a new share certificate with No. 20,001. In the new share certificate No. 20,001, you should write all the distinctive numbers (not continuously) 101-105, 116-120, 131-135 and in the total shares "15" will be written.