Depending on what RDBMS you are using:
SQL SERVER:
SELECT TOP 3 COUNTRY, count(*)
FROM DRUG_SEIZURE
WHERE COUNTRY IS NOT NULL
GROUP BY COUNTRY
ORDER BY count(*) DESC
MySQL:
SELECT COUNTRY, count(*)
FROM DRUG_SEIZURE
WHERE COUNTRY IS NOT NULL
GROUP BY COUNTRY
ORDER BY count(*) DESC
LIMIT 3
Oracle:
SELECT *
FROM (
SELECT COUNTRY, count(*)
FROM DRUG_SEIZURE
WHERE COUNTRY IS NOT NULL
GROUP BY COUNTRY
ORDER BY count(*) DESC
) mr
WHERE rownum <= 3
ORDER BY rownum;