Below is my trigger:
trigger opportunityinsertupdate on opportunity(before insert,before update) {
    if(trigger.isInsert) {     
        for(opportunity o : trigger.new) {
            if(((o.Record_Type_Name__c == 'NC Power')||(o.Record_Type_Name__c == 'NC Oil & Gas')) && (o.FS_Included__c == false) && (o.Super_Region__c == 'Asia/India')) {
                o.stagename = 'Sales Lead';
                o.amount = 1;
                o.CurrencyIsoCode = 'USD';
                o.Target_ShipDate__c = o.Target_ShipDate__c.addmonths(3);
            }
        }
    }
   
    if(trigger.isUpdate)  {
        for(opportunity o:trigger.new){
            if(((o.Record_Type_Name__c == 'NC Power')||(o.Record_Type_Name__c == 'NC Oil & Gas')) && (o.FS_Included__c == false) && o.Super_Region__c == 'Asia/India' && o.Check__c == false) {
                o.stagename = 'Sales Lead';
                o.amount = 1;
                o.CurrencyIsoCode = 'USD';
                o.Target_ShipDate__c = o.Target_ShipDate__c.addmonths(3);
                o.Check__c = true;
            }
       
        }
    }
}

But i was facing a problem that when i insert a record the shipdate date is adding 6 months
and check__c field is setting to true..
But my requirement is a record is inserted the shipdate should add 3 months only with exiting shopdate
and when any record is updated then shipdate should add 3 months and check box is set to true..
but with my trigger when i am inserting also the check box is seeting to true and shipdate is adding to 6 months..
can any suggest me to solve my trigger please
thanks in advance..