Sudbury's young team started well and in the early stages Harwich were under some pressure with Shrimpers goalkeeper Jack Lines having to be alert to the danger.
Harwich came back with Archie Corner's cross whizzing just wide of the far corner of the top of the net.
With 27 minutes gone, Harwich gained the initiative, taking the lead when an attack forced Sudbury's keeper to advance and Jaidan Smith took advantage to neatly slot the ball into the net.
It was a good half for Harwich and 8 minutes before the break they scored their second goal when an effort came back out to Smith and from outside the penalty box he netted with another good finish.
Harwich manager Michael Clarke wanted to see all of his squad in action and early in the second half he introduced 4 of his substitutes to the game.
Sudbury were mounting some well-worked moves and the half was evenly contested.
The skilful Ewan Livingston made an eye-catching run through the visitors' defence and looked likely to increase the Shrimpers' lead but when he reached a scoring position he was thwarted by a really good block tackle.
With 5 minutes remaining, Smith scored his and Harwich's third goal when he ran through to slot home a well-taken finish.
After the final whistle, Freddy Bethell was presented with the Harwich man-of-the-match award and Smith also received an award for his hat-trick.
Harwich team: Jack Lines, Tony Clarke, Joe Simmons, Oscar Warnes, Archie Corner, Batuhan Macit, Ewan Livingston, Freddy Bethell, Jaidan Smith, Cody Harrison and Alex Pryke. Substitutes: Sol Griffiths, Archie Rogers-Bell, Rory Breen, Finley Caro and Harvey Smith (all used).