[SalesForce] Salesforce Validation Rule Only Create 1 Record a day

As mentioned by @Stygon I have used roll-up summary field to get the count and used a validation rule to show message if the count exceeds 3.

You will have a trigger on the job posting object. When inserted (before insert), you should check if the position for that job posting already has other job postings. If yes, then you should get the time of the last one inserted. If the date equals today, then you cannot insert the object (using .addError method).

It would probably look like this:

trigger on Job_Posting__c (before insert) {
    if (Trigger.isBefore && Trigger.isInsert) {
        List<Id> positions_ids = new List<Id>();

        // get the positions ids
        for (Job_Posting__c j : Trigger.new) {
            positions_ids.add(j.Position__c);
        }

        // query all job postings for those positions
        List<Job_Posting__c> other_jobs = [SELECT Id, CreatedDate, CreatedById FROM Job_Posting__c WHERE Position__c IN positions_ids];

        for (Job_Posting__c j : Trigger.new) {
            for (Job_Posting__c existent_record : other_jobs) {
                // check if we are comparing two records for the same position
                // if the created date for the existing record equals the current date
                // (that is, the insertion date for the record on Trigger.new), then 
                // we can send that error to the user.
                if (j.Position__c == existent_record.Position__c && j.CreatedById == UserInfo.getUserId()) {
                    Date existent_record_created_date = existent_record.CreatedDate.date();

                    if (existent_record_created_date == Date.today()) {
                        j.addError('Cannot create job position because there\'s another one created on this date.');
                    }
                }
            }
        }
    }
}