ARM Template - Conditionally add to an Array

If I Understand it correctly then you want to conditionally execute the VirtualNetworkRule depending on the string parameter externalsubnet1 . If parameter value is present then x1 will be executed and if value is blank then x2 and x3 will be executed. In that case you can use the below template:

Template:

{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {        
        "externalSubnet1": {
            "type": "string"
        }
    },
    "variables": {
        "SQLServerName": "ansserver",
        "Subnet1": "/subscriptions/xxxxxxxxxxxxxxxxxxxxxxxxx/resourceGroups/ansumantest/providers/Microsoft.Network/virtualNetworks/ansuman-vnet/subnets/default",
        "Subnet2": "/subscriptions/xxxxxxxxxxxxxxxxxxxx/resourceGroups/ansumantest/providers/Microsoft.Network/virtualNetworks/ansuman-vnet/subnets/subnet1"
    },
    "resources": [{
            "name": "[variables('SQLServerName')]",
            "type": "Microsoft.Sql/servers",
            "location": "East US",
            "apiVersion": "2015-05-01-preview",
            "properties": {
                "administratorLogin": "ansuman",
                "administratorLoginPassword": "password",
                "version": "12.0",
                "publicNetworkAccess":"Enabled"
            },
            "resources": [
                {   
                    "condition":"[not(empty(parameters('externalSubnet1')))]",
                    "type": "virtualNetworkRules",
                    "apiVersion": "2021-05-01-preview",
                    "name": "[concat(variables('SQLServerName'),'test1')]",
                    "dependsOn": ["[resourceId('Microsoft.Sql/servers', variables('SQLServerName'))]"],
                    "properties": {
                        "virtualNetworkSubnetId": "[parameters('externalSubnet1')]",
                        "ignoreMissingVnetServiceEndpoint": false
                    }
                }, {
                    "condition":"[empty(parameters('externalSubnet1'))]",
                    "type": "virtualNetworkRules",
                    "apiVersion": "2021-05-01-preview",
                    "name": "[concat(variables('SQLServerName'),'test2')]",
                    "dependsOn": ["[resourceId('Microsoft.Sql/servers', variables('SQLServerName'))]"],
                    "properties": {
                        "virtualNetworkSubnetId": "[variables('Subnet1')]",
                        "ignoreMissingVnetServiceEndpoint": false
                    }
                }, {
                    "condition":"[empty(parameters('externalSubnet1'))]",
                    "type": "virtualNetworkRules",
                    "apiVersion": "2021-05-01-preview",
                    "name": "[concat(variables('SQLServerName'),'test3')]",
                    "dependsOn": ["[resourceId('Microsoft.Sql/servers', variables('SQLServerName'))]"],
                    "properties": {
                        "virtualNetworkSubnetId": "[variables('Subnet2')]",
                        "ignoreMissingVnetServiceEndpoint": false
                    }
                }
            ]
        }
    ]
}

Output: