WooCommerce 3.0+ is a really major update and is much more strict than before. A lot of things have changed and most of custom code used for WooCommerce version 2.6+ will need to be updated.
See the related articles to WooCommerce Development blog
Here in your case you need to update all woocommerce templates in your theme located in your customtheme (folder) > woocommerce (sub folder).
To do that, you will have to replace all listed files from the woocommerce plugin "templates" folder to your theme woocommerce (sub folder):
woocommerce/templates/cart/cart.php => customtheme/woocommerce/cart/cart.php
woocommerce/templates/checkout/form-checkout.php => customtheme/woocommerce/checkout/form-checkout.php
woocommerce/templates/checkout/review-order.php => customtheme/woocommerce/checkout/review-order.php
woocommerce/templates/checkout/thankyou.php => customtheme/woocommerce/checkout/thankyou.php
woocommerce/templates/content-product.php => customtheme/woocommerce/content-product.php
woocommerce/templates/loop/loop-start.php => customtheme/woocommerce/loop/loop-start.php
woocommerce/templates/loop/pagination.php => customtheme/woocommerce/loop/pagination.php
woocommerce/templates/myaccount/form-edit-address.php => customtheme/woocommerce/myaccount/form-edit-address.php
woocommerce/templates/single-product/add-to-cart/variable.php => customtheme/woocommerce/single-product/add-to-cart/variable.php
woocommerce/templates/single-product/product-image.php => customtheme/woocommerce/single-product/product-image.php
woocommerce/templates/single-product/product-thumbnails.php => customtheme/woocommerce/single-product/product-thumbnails.php
woocommerce/templates/single-product/short-description.php => customtheme/woocommerce/single-product/short-description.php
woocommerce/templates/single-product/tabs/description.php => customtheme/woocommerce/single-product/tabs/description.php
woocommerce/templates/single-product/title.php => customtheme/woocommerce/single-product/title.php
But keep a copy of that old template before, as you will need to replace in the new templates all changes that you have made.
See this related documentation: Template Structure + Overriding Templates via a Theme
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