The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
2. Use UNION ALL instead of UNION (query line: 25): Always use UNION ALL unless you need to eliminate duplicate records. By using UNION ALL, you'll avoid the expensive distinct operation the database applies when using a UNION clause.

Optimal indexes for this query:

ALTER TABLE `tablename1` ADD INDEX `tablename1_idx_active` (`Active`);
ALTER TABLE `tablename2` ADD INDEX `tablename2_idx_active` (`Active`);
ALTER TABLE `tablename3` ADD INDEX `tablename3_idx_active` (`Active`);

The optimized query:

SELECT
        tablename1.Id,
        tablename1.productName,
        tablename1.Largeimagepath,
        tablename1.Discount,
        tablename1.Price,
        tablename1.Image 
    FROM
        tablename1 
    WHERE
        tablename1.Active = 1 
    UNION
    SELECT
        tablename2.Id,
        tablename2.productName,
        tablename2.Largeimagepath,
        tablename2.Discount,
        tablename2.Price,
        tablename2.Image 
    FROM
        tablename2 
    WHERE
        tablename2.Active = 1 
    UNION
    SELECT
        tablename3.Id,
        tablename3.productName,
        tablename3.Largeimagepath,
        tablename3.Discount,
        tablename3.Price,
        tablename3.Image 
    FROM
        tablename3 
    WHERE
        tablename3.Active = 1