Boosted by an early penalty from Liam Walshe, his 21st goal of the season, Grouville went on to net again through Chris Carter and with Trinity then having defender Stephen Jones sent off, last year’s beaten finalists had a mountain to climb to get back into the game but to their credit they worked really hard through to the finish.
Luc Le Mière hit Grouville’s third in the second half with a low left-foot shot off the inside of a post, just seconds after Trinity’s Dave Le Roux burst through only to be denied by an excellent tackle by John Kiersey.
Grouville’s victory was their 12th in 13 games while Trinity, who were also beaten in the league 3-0 by Grouville on Saturday, slumped to their fifth defeat in six matches.
Grouville joint-manager Ian Horswell said: ‘We’re delighted to keep our good run going and reach a cup final, we’ve had two very tough games against Trinity in four days.
‘Casey Hickling made a couple of good saves for us, Trinity are not getting the rub of the green at the moment and I knew they would have good periods because they have some fine players.
‘We had to be aware of that and be as solid and organised as possible when it happened and I think we achieved that.’ Trinity and Island player Dave Le Roux said: ‘Silly mistakes are costing us goals and that is making it more difficult for us every time we go behind.
‘It happened on Saturday with an own goal and last night we conceded a penalty inside ten minutes after we’d had a bright start with some good chances.
‘It’s great to see Darren Swanson back playing again after injury, I don’t think he’s played since early September.
‘Unfortunately we now have a habit of losing and it’s really deflating.
I thought we played better with ten after Stephen Jones was sent off and I’m confident we can turn things around soon.
‘We’re in a fight at the bottom of the league but we’re still in two cup semi-finals.
On our day we have the ability to beat all teams but our immediate attention now must be to tighten up in defence first.’ Trinity defender James Hayward had five stitches inserted into a head wound after a collision with an opponent last night.
The final between Rozel Rovers and Grouville will be played at Springfield on Thursday 26 April, kick-off 7.30 pm.
Trinity F&SC: Jimmy Styles; James Hayward, Jon Fitzmaurice, John Beatson, Stephen Jones; John Rutter, Ashley Dodd, Darren Swanson, Stephen McCarter; Jimmy Reilly, Dave Le Roux.
Subs used: Gerard Meadows, Paul Beckett, Graeme Ling.
Sent off: Jones.
Cautioned: Jones, Fitzmaurice, Meadows.
Grouville: Casey Hickling; Rob Cameron, Jacek Bogdan, Russell Le Feuvre, Craig Northedge, John Kiersey, Luc Le Mière, Liam Walshe, Alex Jefferies, Peter Warrell, Chris Carter.
Subs used: James Ollivro, Toni Cardoso, Paul Crompton.
Officials: Andy Legg, Mark Le Cornu, Chris Gouyette.