I don’t think tokenizer handles this case directly.
You could directly join the sentences using [SEP] and then encode it as one single text.
tok = BertTokenizer.from_pretrained("bert-base-cased")
text = "sent1 [SEP] sent2 [SEP] sent3"
ids = tok(text, add_special_tokens=True).input_ids
tok.decode(ids)
=> '[CLS] sent1 [SEP] sent2 [SEP] sent3 [SEP]'